Litten is the Fire-type starter. It can be received from Professor Kukui on Route 1's main route.
Beltigre makes its grand return to the Alola region with a couple of new tools, some new matchups, and a good amount of usage that lasts throughout the entire game. While not exactly the fastest fighter, it packs an incredible punch while sporting a decently-sized movepool, which is only made better through the addition of the Move Tutors. This along with its balanced defensive stats makes it one of the most viable, if not the most viable, Fire-type in the story.
Hau (Route 1): So much for "Ultra". Spam Ember to win.
Hau (Iki Town): Pichu is as much of a joke as it was back in the original games - Thunder Shock is minimal damage, Charm doesn't affect your only STAB attack at this point, and Ember 2-3HKOs. But as easy as Pichu is, Rowlet is even easier - once again, use Ember to win.
Ilima (Hau'oli City, Normal-type): You must take the absolute greatest caution if fighting the Yungoos - both your Ember and its Tackle are 3HKOs, and while you outspeed and win without outside interference, you must note two things: Ilima has a Potion, and there are always a threat of critical hits - fight only if necessary. Stay away from Smeargle entirely. Its Water Gun 3HKOS, while your Ember is unfortunately stuck in 4-5HKO range.
Totem Gumshoos/Raticate (Verdant Cavern): If you've equalized yourself to their levels at 12... this fight won't work. Gumshoos and Raticate cause so much damage with their Bite attacks, and their ally summons will not make it any easier. However, if you do overlevel yourself to 16, you will learn Double Kick, which 2HKOS either Totem, although overleveling that much requires dedication.
Dulse/Soliera (Seaward Cave): Keep away from this fight entirely. Furfrou outspeeds and can 2HKO you with Headbutt, while you're stuck in 6HKO with Ember. Even at level 16, when you learn Double Kick, you still lose the fight because of your lack of speed against it.
Hau (Route 3): Hau leads with Rowlet, and that makes Litten a very useful lead, even though Ember is only a 3HKO, because Rowlet's attacks can only 5HKO. Noibat is a simple fight if you're healed up - your Scratch 4HKOs, while his Noibat is stuck in 5HKO range, so since you outspeed, you will win this matchup. Pikachu, shockingly, was nerfed from Sun and Moon, because of the lack of Electro Ball in its moveset. It now is stuck with Thunder Shock, which is considerably weaker and lacks the ability to KO you in any more than 6 turns. Meanwhile, your Ember 4HKOs it, making this fight in general fairly simple.
Hala (Iki Town, Fighting-type): Stay right away from Machop - Revenge can and likely will OHKO. Makuhita is a bit easier, but still very strong - it can lead with Fake Out, and its Arm Thrust 3HKOs on average, while your Fire Fang is also stuck in 3HKO range. As for Crabrawler, staying away is the best option - an All-Out Pummeling will certainly OHKO Litten, while your Fire Fang will 4HKO at best.
Hau (Paniola Town):Hau's a filthy cheater that's what he is Hau leads with his Dartrix, which appears to have evolved a whole level early. Despite this advantage, he still loses to Litten, which outspeeds Dartrix and can 2HKO with Fire Fang - even a potential Breakneck Blitz won't be enough to turn the fight in his favor. Pikachu is a little more dangerous, because it outspeeds and can 3HKO with Electro Ball, but tolerable if you have full health, because your Fire Fang will likely 2KHO. Noibat is a cakewalk - even though your attacks can only 4HKO it, its Bite will only 6HKO at best. Eevee is 2HKOed by Double Kick provided you don't get hit by Baby-Doll Eyes - even then, it won't be able to cause significant damage to you before you inevitably kill it. One more thing of note is that if you have leveled up to 17 - Torracat's evolution level - the piece about "Pikachu being a little dangerous" is tossed out the window; Torracat can 2HKO with Fire Fang, you gain a speed advantage over it, and its Electro Ball is dropped to a 4-5HKO, making the whole fight extremely clean for Torracat.
Gladion (Route 5): Don't let Gladion's first Pokemon fool you - it's not a Zubat, it's a Zorua. Smack it around with Double Kick to 2HKO it easily. When he sends out his actual Zubat, Fire Fang will be capable of 2HKOing it easily, while its Wing Attack is stuck in 4HKO range. At full health, you should be able to defeat Type: Null - your Double Kick 3HKOs and you outspeed, while its Tackle can only 4HKO, but if you're weakened from the other Pokemon, you may want to switch out or take a turn to use a Potion.
Totem Araquanid (Brooklet Hill): Bubble will OHKO Torracat. Stay away.
Dulse/Soliera (Paniola Ranch): In theory, you should defeat Poipole if you are at full health - although both your Fire Fang and its Venoshock are 3HKOs, you outspeed. Still, be wary of critical hits.
Battle Royal (Royal Park): Your best bet is to target Hau's Dartrix with Fire Fang while staying wary of Type: Null, which hits hard, and the Royal's Rockruff, which can deal super-effective damage to you.
Totem Marowak (Wela Volcano Park): Torracat doesn't have the strength to deal with the Totem, which can hit with a Thick Club-boosted Brick Break, or the Salazzle, which can poison and use Venoshock. Keep it away from this fight.
Totem Lurantis (Lush Jungle): Colress gave you Flame Charge earlier for a reason. If you taught it to Torracat - and you should have, even if only for this fight - use it on turn 1. This will raise your speed to a point just above the Totem's. On Turn 2, if you use your new Inferno Overdrive Z-Move against the Totem - a Z-Move originated from Fire Fang - it should KO the totem nice and cleanly. If you can't do this, the fight will be tricker, but tolerable. However, if the Totem Lurantis summons a Kecleon and you can't defeat Lurantis within the next turn, switch out - Kecleon's Ancient Power will hurt.
Plumeria (Akala Outskirts): Golbat is a bit too tough for Torracat - its Wing Attack 3HKOs, and it outspeeds, while your Fire Fang is stuck in 4HKO range. Salandit is a little easier, but not by much - although your Bite can 3HKO, Salandit knows Dragon Rage and it outspeeds, so if you have 80 HP or less, let something else deal with it.
Olivia (Ruins of Life, Rock-type): Anorith can be dealt with, but it's risky. Only fight it if your Attack stat is at least 64 and your Speed is at least 71, because then, and only then, will you be able to outspeed and 2HKO with Fire Fang. Anything else, and Anorith will 2HKO you with Smack Down. Lileep is a bit out of Torracat's range to battle - it's too bulky to fight even with Double Kick, and its Ancient Power can 2HKO. Lycanroc should be fought under no circumstances - if it decides to click Continental Crush, it's all over for Torracat.
Nihilego (Aether Paradise): Torracat's gonna have trouble here - let something else take it out.
Hau (Malie City): Dartrix is safe, but not as safe as one would like - although you outspeed and your Fire Fang 2HKOs, its Pluck can 2HKO as well. Still, you should be able to win. Don't fight Vaporeon - just trust me on this. Noibat's as easy as it ever was - you should be able to 2HKO with Bite, while its Wing Attack is stuck in 4HKO range. Tauros is just too tough for Torracat, with its Tackle having a high chance to 2HKO, so keep away from it. Finally, we have Raichu. Although your Bite is a solid 2HKO, Raichu outspeeds and can 2HKO with Psychic, so stay away from it.
Totem Togedemaru (Hokulani Observatory): A Torracat at equal level to Togedemaru will have trouble winning and should stay away from the battle, as the Totem's Zing Zap can 2HKO. However, if you level up to 34 and evolve into Incineroar, the difficulty of the fight goes down greatly, especially if you hold a Firium Z. On turn 1, the Totem may lead with Spiky Shield, giving you the opportunity to use an X Attack to move against the Totem's defense boost. At +2 Attack, Incineroar's Inferno Overdrive attack should 2HKO or even OHKO the Totem, even with its Defense boost. As for the Totem's attacks, Zing Zap will only 4HKO Incineroar, and its other moves do even less, making Incineroar ideal to deal with Togedemaru.
Guzma (Malie Garden): The big bad Golisopod is far, far too tough for Beltigre here - its Razor Shell has a chance to OHKO Incineroar, so keep as far away from it as possible. Masquerain is easier, but only if you have Firium Z in hand. Use Inferno Overdrive to OHKO it, but if you can't launch a Z-Move, switch out, as Bug Buzz has a chance to 2HKO, and it outspeeds.
Totem Mimikyu (Thrifty Megamart): Totem Mimikyu may be a little too strong for Incineroar in this battle - its Play Rough has a good chance to 2HKO. If you're hellbent on battling it, however, you can stand a chance with healing, X-items, the tutor move Iron Head, and a healthy dosage of Z-Power. Drop an X-Defense, break its disguise, then use a Corkscrew Crash off of Iron Head, allowing you to potentially OHKO it. Even if you fail to OHKO, it will be in a position to fall in one more attack. But be warned - Mimikyu is fast, and will certainly outspeed your Incineroar. Play this path correctly and safely, or don't play it at all.
Plumeria (Route 15): Darkest Lariat will 2HKO Golbat, while its Wing Attack only has a small chance to 4HKO, making that portion of the fight safe to deal with. When dealing with Salazzle, your Malicious Moonsault Z-move will certainly OHKO it, although make sure you have at least 50% of your HP remaining, because Sludge Bomb has about a 33% chance to 2HKO you, and Salazzle will outspeed and hit you before you can hit it.
Guzma (Shady House): Golisopod is off-limits - Razor Shell still has that chance to OHKO. Masquerain is somewhat risky thanks to Intimidate, which keeps your Fire Punch from securing an OHKO, but if you choose to run Firium Z rather than Incinium Z for this fight, an Inferno Overdrive will break through that Intimidate and OHKO Masquerain before it can take you down. Pinsir is just too dangerous - it outspeeds, and Storm Throw will 2HKO you.
Gladion (Aether House): Use Darkest Lariat immediately, as Gladion actually sends out Golbat first, and not Zorua like last time. This will 2HKO Golbat. If the Type: Null he sends out is level 42, that is actually a Zoroark, and you should use Leech Life, which will greatly heal you and even possibly OHKO it. Once he sends out his real Type: Null (which can be identified by it being level 43), use Malicious Moonsault to bring it down to critical condition, then finish it off with a move of choice. But make sure you're healed up, because Null's Crush Claw can 3HKO Incineroar.
Nanu (Malie City): Sableye will likely lead with Fake Out, so instead of wasting your first turn, use an X Attack. If you do so, you will be able to OHKO Sableye with Darkest Lariat. At +2, Krokorok is OHKOed by Leech Life, which will restore most of the damage taken from Earthquake. Be careful, though - a crit Earthquake will likely OHKO Incineroar. The Persian is 2HKOed with +2 Leech Lifes, and you can restore any lost health from its Power Gems through spamming it. All in all, as long as you use that X Attack during Sableye's Fake Out turn, this fight will be a piece of cake.
Faba (Aether Paradise): Darkest Lariat will likely OHKO Hypno. You may use Malicious Moonsault to secure the faint, however.
Faba (Aether Paradise, tag battle with Hau): Faba is a piece of cake as long as you take out that Ledian before it sets up Reflect. You do OHKO it with Fire Punch, but it outspeeds, and if it sets it up, you will have a bit of a problem with damage, and you might want to switch out to another Pokemon - preferably a Special Attacker. Without the Reflect, however, this fight is more than doable. The Aether Employee's Pupitar is OHKOed by Brick Break, and Slowbro will be 2HKOed by Darkest Lariat, but you may use your Malicious Moonsault immediately on it should you wish, because both Hypno and Bruxish will likely be OHKOed by Darkest Lariat.
Guzma (Aether Paradise): Treat Golisopod as you would every other encounter with it: keep as far away as possible. Masquerain is 2HKOed by Fire Punch, although this still isn't safe, because its Bug Buzz has a chance to 2HKO as well, and it outspeeds - it is recommended to keep back from it. Vikavolt is doable for Incineroar, whether you decide to use Inferno Overdrive, which will OHKO, or if you use Malicious Moonsault, which won't quite OHKO, but will leave Vikavolt in position to be defeated with a Fire Punch the following turn. Finally, stay away from Pinsir, because Storm Throw hurts, and Stone Edge hurts even more.
Dulse/Soliera (Aether Paradise): If you're battling Soliera in Ultra Moon, she will lead with an X Sp. Def. on their Poipole. This is their downfall, because your Malicious Moonsault will OHKO Poiple before it even gets an attack off. Dulse in Ultra Sun, however, leads with an X Defense. That, in conjunction with his Poipole's full investment in physical bulk, makes it a bit too much for Incineroar to handle.
Lusamine (Aether Paradise): Clefable is far too bulky and just a little too heavy-hitting for Incineroar to deal with, so it would be wise to keep back from it. Lilligant is much, much easier, being 2HKOed by Fire Punch. Alternatively, feel free to use a Firium Z in this fight to launch an Inferno Overdrive to secure the OHKO. You can deal with Lopunny by 2HKOing with Brick Break, making dealing with it fairly simple. Stay away from Milotic - Hydro Pump hurts - and Bewear - Drain Punch hurts.
Dulse/Soliera (Vast Poni Canyon): This fight is essentially the exact same as the one in Aether Paradise. If you're playing Ultra Moon, OHKO Poipole with Malicious Moonsault. If you're playing Ultra Sun, let something else deal with it.
Totem Kommo-o (Vast Poni Canyon): Nope. Incineroar does not want to deal with a Fighting-type pseudo-legendary with a +1 boost to their attack.
Dusk Mane Necrozma (Altar of the Sunne, Ultra Sun only): Necrozma can hardly hurt Incineroar at all, although it can annoy you quite a bit with its Morning Sun. You will want to immediately lead off with your Malicious Moonsault to weaken it to critical condition, then finish it off with either Darkest Lariat or Fire Punch. Alternatively, use an X Attack to secure the Z-Move's OHKO and just ignore the threat of Morning Sun.
Dawn Wings Necrozma (Altar of the Moone, Ultra Moon only): If Dusk Mane Necrozma was easy, Dawn Wings is even easier. You will certainly OHKO with Malicious Moonsault.
Ultra Necrozma (Megalo Tower): And if you thought both Dusk Mane Necrozma and Dawn Wings Necrozma were easy, then Ultra Necrozma... wait, no. This is a terrible matchup, even if you did make the hellish grind to level 60. No matter what you do, Power Gem will range from either a likely OHKO to a certain OHKO.
Mina (Seafolk Village, Fairy-type): Fire Punch is a 2HKO on Mawile after Intimidate, although be careful, as its Play Rough will 3HKO. Granbull is too dangerous for Incineroar, as it outspeeds and can 2HKO with Earthquake or Stone Edge. As for Ribombee, only battle it if you're switching in onto it, as its Z-Move can take out up to 80% of Incineroar's health in one shot. However, if you've got enough health to deal with its attack, your Fire Punch will OHKO Ribombee.
Ilima (Hau'oli Cemetery, Normal-type): Both your Brick Break and Gumshoo's Hyper Fang will 3HKO each other, but Incineroar will outspeed it - feel free to fight it if you're confident that a crit won't occur. Smeargle's a pushover, even with its Surf - OHKO with Brick Break. Malicious Moonsault will OHKO Ilima's Komala, but only try this if you're at full health - it can outspeed you and deal up to 90% of your health in damage with its Breakneck Blitz attack.
Mallow (Lush Jungle, Grass-type, Ultra Sun only): Darkest Lariat 2HKOs Trevenant - not even its Horn Leech can save it from that fate. As for Shiinotic, you will OHKO it with an Inferno Overdrive off of Fire Punch, although if you didn't remove your Incinium Z for this fight, Fire Punch is still a solid 2HKO. Just be wary of its Spore attack, as well as the fact that Moonblast can 3HKO depending on the damage rolls. When dealing with Tsareena, it has a 20% to 2HKO you with Low Sweep, although that damage output is partially remedied with Leech Life, which itself has a 90% to 2HKO Tsareena. All in all, a very good matchup for Beltigre.
Lana (Lush Jungle, Water-type, Ultra Moon only): This is not a good matchup for Incineroar here. Lanturn's Hydro Pump can OHKO, and Araquanid's Hydro Vortex will certainly OHKO. Even though Cloyster doesn't know any water-type attacks, its defenses are too much for Incineroar, forcing even its super-effective moves down to 4HKO range, and because of its Skill Link, Spike Cannon is a 4HKO as well.
Kiawe (Wela Volcano Park, Fire-type): Darkest Lariat will take down Arcanine eventually, even after taking an Intimidate, but it's an uphill fight and not recommended for Incineroar. Do not fight Talonflame - its Brave Bird will kill Incineroar in two shots, and it outspeeds. If you want to fight Marowak here, use your Malicious Moonsault on turn 1. Even though Marowak will outspeed, it can't outright kill you with Bonemerang unless you're reduced to below half health, while your super-effective Z-Move will OHKO it.
Sophocles (Hokulani Observatory, Electric-type, Ultra Sun only): You want to make this fight extremely easy? TM78 - Bulldoze - is on sale for $10000 at Konikoni City's TM shop. Swap it out for Brick Break in this fight, just temporarily. Doing so will allow you to OHKO Togedemaru after taking a Zing Zap. You will also 2HKO Magnezone, but make sure that you have a base Speed stat of 78 or higher, or else Magnezone will outspeed you and possibly 2HKO you in return with Discharge. Keep away from Golem entirely - Stone Edge is a guaranteed OHKO.
Nanu (Aether House, Dark-type, Ultra Moon only): Sableye lacks Fake Out in this fight, meaning that you will take a hit from Power Gem if you decide to set up an X Attack in this battle. However, do not be afraid, as once you use an X Attack, Sableye is OHKOed by Darkest Lariat. Absol is a pushover due to lack of coverage - OHKO with a +2 Leech Life to get back that lost health. As for Persian, spam Leech Life. At +2 Attack, you will 2HKO and make up any lost health from its own Power Gem attacks.
Totem Ribombee (Seafolk Village): Don't try this fight. Its +2 boost to all stats, Occa Berry, and neutral coverage to Incineroar's typing make it impossible for Incineroar to battle it on its own.
Hapu (Exeggutor Island, Ground-type): No. This fight is very bad for Incineroar. The only thing it can even think about doing in this battle is coming out, using Malicious Moonsault on her lead Golurk to OHKO, then get out of there immediately afterwards. Other than that, Incineroar cannot handle this battle.
Gladion (Mount Lanakila): Crobat is too dangerous to tackle head-on - both your Darkest Lariat and its Acrobatics have the same chance to 2HKO, but Crobat outspeeds. Meanwhile, Gladion's Zoroark planning skills have become very clever - he now has it disguised as Lucario in this battle. Both are level 53, making it impossible to tell until it decides to attack. Furthermore, the real Lucario's Aura Sphere can deal 70% of Incineroar's health in damage immediately, while Zoroark is stuck in 4HKO range with Hyper Voice, so deciding whether or not to have Incineroar out for this fight is tricky. To test the waters and make your decision safer, though, there is one move you can use: Fire Punch. Fire Punch has a chance to OHKO the real Lucario, and even if it's not and you hit the illusioned Zoroark instead, you still deal 50% of its health in damage with a minimal damage roll, and you can give yourself a small heal the next turn by using Leech Life. As for Silvally, keep away from it. It's running a Water Memory, and its Multi-Attack will hurt a lot.
Elite Four Molayne (Pokémon League, Steel-type): When dealing with Klefki, it may lead with Reflect - do not let this deter you. Use Fire Punch until it is down to critical condition, which will be around three hits, then use Brick Break to destroy its shield. Other than the annoyance with Reflect and Thunder Wave, Klefki can't damage Incineroar outside of Flash Cannon, which can only 8HKO at best. Bisharp is manageable with Brick Break, which will 2HKO even if the Reflect was still up, while its X-Scissor is stuck in 4HKO range. You may want to stay away from Magnezone - even though you outspeed, your Fire Punch doesn't even guarantee a 2HKO, while Magnezone's Thunderbolt can 2HKO with high damage rolls - and that's not even accounting the fact that the aforementioned Bisharp can use Metal Sound to cut your Special Defense in half, turning that 2HKO chance into an OHKO chance. Metagross has a high chance to be OHKOed with Malicious Moonsault, and even if you miss the OHKO and it attempts to kill you with Hammer Arm - which can 2HKO - you will be able to defeat it before it gets the second strike off. Dugtrio is dangerous - even though you will certainly OHKO it with Fire Punch, it can OHKO you with Earthquake, and if its Fissure lands, it's all over - battle at your own risk.
Elite Four Olivia (Pokémon League, Rock-type): This battle just doesn't work out in Incineroar's favor. Armaldo hits too hard, Cradily and Probopass are too bulky, Gigalith combines both of those factors, and Lycanroc has a Z-Move. Keep Incineroar away from the fight.
Elite Four Acerola (Pokémon League, Ghost-type): Unlike Olivia, Acerola is a nearly-perfect matchup for Incineroar in every way. Banette is OHKOed by Darkest Lariat, very easy. Darkest Lariat will 2HKO Drifblim, while the best it can do is Baton Pass some Amnesia boosts or cause minimal damage with Ominous Wind - just make sure it doesn't pass any omniboosts, and make sure you're above 25% health when you KO it, or else it will kill you with Aftermath. Darkest Lariat or Fire Punch will 2HKO Dhelmise, while it won't be able to do much to you at all - even its best attack, Slam, will only 5HKO. Darkest Lariat OHKOs Froslass, and even its powerful Blizzard can only 5HKO. Finally, there's Palossand. Even though Earth Power's 2HKO chance may seem scary, there are two things you have that frighten even this Ghost: you outspeed, and you have Malicious Moonsault. Use your Z-Move to OHKO it.
Elite Four Kahili (Pokémon League, Flying-type): Braviary is too tough to take on alone - Brave Bird will 2HKO Incineroar. Hawlucha is even more dangerous, with its Flying Press having a chance to take out 90% of Incineroar's health with one hit. Oricorio can be dealt with by OHKOing it with Malicious Moonsault, but if it uses Feather Dance before your attack, switch out immediately afterwards. Mandibuzz is too bulky for Incineroar to fight, and its Bone Rush can 3HKO, so stay back from it. Finally, Skill Link Toucannon with Rock Blast and Z-Beak Blast? That's a recipe for disaster.
Hau (Pokémon League): Raichu's Focus Blast may seem dangerous in the immediate, but it can't do more than 2HKO, while your Darkest Lariat OHKOs. That, in conjunction with its shaky accuracy, makes Raichu moderately safe to deal with. Stay as far away from Vaporeon as possible - Hydro Pump is a guaranteed OHKO on Incineroar. Tauros is also dangerous. Its Intimidate prevents you from securing anything more than a 3HKO, while its Earthquake will 2HKO you easily. Noivern may have finally reached its final evolution, but it's still a moderately simple battle when you use your Z-Move. Malicious Moonsault has a 50% chance to OHKO, while Noivern's Dragon Pulse is stuck at 3HKO range. Crabominable OHKOs with Close Combat, so stay away from it. Finally, there's Decidueye. Decidueye is 2HKOed with Darkest Lariat, and even its secret weapon in the form of Smack Down isn't enough to save it from its inevitable doom at the hands of best starter.
Post-Game: Ubers are scary. All of them. Rainbow Rocket had the right idea.
Not much has changed from Sun and Moon in regards with TM and Tutor Moves, with two notable exceptions. After evolving into Incineroar, the move tutor on Akala Island will be able to teach Incineroar Fire Punch, a direct upgrade from Fire Fang in terms of power and accuracy. Also, after reaching Ula'ula Island, Incineroar can learn Iron Head from that move tutor, allowing Incineroar to deal with otherwise-untouchable Fairy-types. Otherwise, a lot of the basic movepool options here are the same. As early as Litten, it can learn Leech Life for a useful HP recovery option, Flame Charge for a quick speed boost, and U-Turn for an increase in mobility. Return can provide powerful neutral coverage, but the general support of the other moves generally outclass it. Acrobatics is also an option, but using it requires a sacrifice of Incineroar's very useful Z-Crystal slot. As for lategame tactics as Incineroar, Bulldoze allows you to have some offense against Rock-types while reducing Speed on foes and Brick Break for an accurate Fighting-type attack that shatters shields. Bulk Up is also usable to provide an increase in both power and defense, and should be considered if you're not using X-Items.
Litten should hold a Firium Z for a while in order to use Inferno Overdrive, giving a powerful nuke in most battles until you evolve into Incineroar and learn Darkest Lariat. As soon as you do that, Firium should be swapped out and replaced with Incinium to use Malicious Moonsault. Swapping of these two Z-Crystals can be used to deal with certain matchups differently, such as Guzma or Lusamine, although Incinium Z will be the most common crystal used. There is also an opportunity to use Corkscrew Crash after reaching Ula'ula Island off of Iron Head, allowing you to deal with Totem Mimikyu, but this is fairly limited in its uses after that.
Recommended moveset: Darkest Lariat, Fire Punch/Flare Blitz, Leech Life/U-Turn, Brick Break/Iron Head/Cross Chop/Bulldoze/Bulk Up
Grass-types: Incineroar has a few type-based weaknesses. Fortunately enough, three of them - Water, Ground, and Rock - can be taken care of with a good Grass-type Pokemon. In return, Incineroar can hit Ice and Bug-types super-effectively, types which would otherwise plague Grass-types. Incineroar's Fire-type moves can also hit Steel-types, which Grass-types can hardly touch.
Fairy-types: To take care of Incineroar's last weakness to Fighting-types, a good Fairy-type Pokemon can work wonders, as it is capable of hitting Fighting-types super-effectively. Most Fairy-types also have enough bulk to take a stronger Fighting-type attack upon switching in. In return, Incineroar can hit Steel-type Pokemon that otherwise plague Fairy-types.
Water-types: Incineroar cannot hit other Fire-types super-effectively without running Bulldoze, so a Water-type provides coverage that Incineroar loves during battles against trainers such as Kiawe or Plumeria. Incineroar also complements the Water-type Pokemon in return by dealing with Grass-type Pokemon that pose an immediate threat.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
Litten Line Ratings
What Nature do I want? Anything that cuts Special Attack or raises Attack, with Adamant being the best one. Litten doesn't use Ember too terribly much before learning Fire Fang, and even though Incineroar uses Speed to an extent, its Attack stat benefits more than its Speed stat with a Nature boost.
At what point in the game should I be evolved? Litten should be evolved into Torracat before the first fight against Gladion on Route 5, while you should be an Incineroar at some point before Totem Mimikyu.
How good is the Litten line in a Nuzlocke? In terms of starters, its very good. Primarina might outclass it in terms of sheer strength due to its Special Attack, but other good Water-types are also common and can cover for the Water-type slot that Primarina would hold. Meanwhile, viable Fire-types like Incineroar are pretty rare, which means that choosing Litten as your starter allows you type coverage that otherwise wouldn't be easy to attain. As for its viability, it's pretty good. Although it requires a bit of babying until you reach the Torracat stage, Incineroar maintains consistent use throughout the game, even having the ability to take down an Island Kahuna and an Elite Four member singlehandedly. And heck, its Z-Move by itself makes Incineroar extremely fun to fight with. In short, I definitely recommend using Litten. You won't regret it.
Litten's and Torracat's type matchups:
Weaknesses: Water, Ground, Rock
Resistances: Fire, Grass, Ice, Bug, Steel, Fairy
Neutralities: Normal, Electric, Fighting, Poison, Flying, Psychic, Ghost, Dragon, Dark