Cyndaquil is available as the Fire-type starter in the Johto-based games.
GSC Cyndaquil is a classic example of a "middle ground" starter Pokemon. It is much better than Chikorita, but not nearly as useful as Totodile. As such, Cyndaquil is a great option for players who want a moderately challenging run, but not too challenging. Once fully evolved, Typhlosion has a very wide movepool by GSC standards which can severely dent or outright many foes, having access to Fire Punch, ThunderPunch, or Earthquake. If you want to use a Fire type in GSC, then the Cyndaquil line is definitely your best bet.
Rival (Cherrygrove City): Spam Tackle. Hopefully you're at least at level 7 or 8 at this point, because Totodile is quite capable of taking hits.
Gym #1 - Falkner (Violet City, Flying-type): Basically, you just spam Ember and use Potions when necessary (thankfully, the AI will not use Mud Slap since Gust does more damage). You should be at a pretty high level though since Cyndaquil matches up very well in Sprout Tower.
Gym #2 - Bugsy (Azalea Town, Bug-type): More Ember spam! Quilava has by far the best matchup against Busgy out of the three starters.
Rival (Azalea Town): You can take out his Gastly and Zubat just fine, but watch out for the Croconaw. You need a better response for that.
Gym #3 - Whitney (Goldenrod City, Normal-type): Basically the only thing Quilava can do against Miltank is spam Smokescreen, which is a rather unreliable strategy. However, it can be useful to do so once or twice if you have no other options, as a single Rollout miss will reset its counter and prevent the damage from stacking up. Stomp still does a lot of damage though, so there are undoubtedly risks to this strategy. Clefairy, on the other hand, is fairly easy.
Rival (Burned Tower): You should be able to handle everything just fine except for the usual Croconaw. Spam Ember against everything else.
Gym #4 - Morty (Ecruteak City, Ghost-type): You definitely want to have Flame Wheel for this fight, but it requires a good bit of overgrinding. If you don't have it, Quilava will likely be stuck with needing heals every other turn, if not every turn, until Morty runs out of PP. Ember alone won't make the cut.
Eusine (Cianwood City, Crystal only): If his Drowzee manages to put you to sleep, then watch out for Dream Eater. Haunter's Curse will annoy you, as well as Electrode's Thunder, which is very strong; avoid it if possible.
Gym #5 - Chuck (Cianwood City, Fighting-type): You can probably take out the Primeape, but stay away from the Poliwrath. The only exception is if you overlevel Quilava to 36, when it evolves into Typhlosion and can then learn ThunderPunch. Even then, the matchup is not ideal.
Gym #6 - Jasmine (Olivine City, Steel-type): Just spam Flame Wheel and win. Steelix's Rock Throw won't even come close to OHKOing Typhlosion. If you still have a Quilava with Flame Wheel, however, Steelix is a problem, but the Magnemites should not be.
Rocket Executive battle #1 (Team Rocket HQ): If you have already gotten through the previous couple gyms, then this should be a cakewalk. If not, then be very careful of the exploding Koffing and company. Any Hyper Fang coming from a Raticate will hurt somewhat badly if you can't land the OHKO. Zubat can be handled just fine though.
Rocket Executive battle #2 (Team Rocket HQ): Flame Wheel or Fire Punch everything and you'll be fine. When in low health, remember to heal instead of switching out if you're facing Murkrow, because it has Pursuit.
Gym #7 - Pryce (Mahogany Town, Ice-type): Just Flame Wheel or Fire Punch spam and win. If you already have a Typhlosion, you can even resort to ThunderPunch to dispose of the seals more quickly. You may want to fight this battle before the previous two gyms.
Rocket Executive battle #3 (Goldenrod Radio Tower): Leave this fight alone. Typhlosion might OHKO the Koffing with Fire Punch, but needs to be at a considerably higher level to do so, and a missed OHKO could mean the end of it thanks to their bomb moves.
Rival (Goldenrod Underground): You should be able to take out everything except for that Feraligatr. Again, prepare a better counter for it.
Rocket Executive battle #4 (Goldenrod Radio Tower): Just like the HQ, Fire Punch spam is the way to go, and ThunderPunch should get you rid of the nasty Murkrow before it starts its Pursuit games.
Rocket Executive battle #5 (Goldenrod Radio Tower): ThunderPunch, and Fire Punch for the Koffing, which in this case can't explode. Houndoom's Faint Attack packs a punch somewhat, but is overall okay for Typhlosion's bulk.
Suicune (Bell Tower, Crystal only): This is a terrible idea and you should know it. Even with ThunderPunch, Suicune's BubbleBeam - with possible Rain Dance support, to boot - does a heck of a lot of damage.
Gym #8 - Clair (Blackthorn City, Dragon-type): Stay away from all the Dragon-types, which is everything Clair has. On the upside, Typhlosion has a decent matchup against the gym trainers.
Rival (Victory Road): Again, just use Fire/ThunderPunch and stay away from the Feraligatr. No, Feraligatr won't die to one ThunderPunch, and the risk is not worth it anyway.
Elite Four Will (Indigo Plateau, Psychic-type): Typhlosion should be able to clean sweep with Fire/ThunderPunch, although you probably won't be able to OHKO the Slowbro unless you're overleveled. In that case, use something else for it instead.
Elite Four Koga (Indigo Plateau, Poison-type): You get super-effective coverage on everything due to your amazing Fire/Electric/Ground awesomeness. Hopefully Typhlosion has Earthquake at this point.
Elite Four Bruno (Indigo Plateau, Fighting-type): You can probably take on everything except for Machamp, but it is probably a better idea to use a teammate with a type advantage against Fighting, especially since the Hitmons are very specially defensive. You can probably OHKO Hitmonlee with Earthquake though, its Defense is ridiculous.
Elite Four Karen (Indigo Plateau, Dark-type): Typhlosion can take on everyone just fine, but the Umbreon is super bulky and super annoying. Prepare X Accuracy and healing items if you plan on fighting it. The rest goes down to either of the Punches.
Champion Lance (Indigo Plateau, Flying-type): ThunderPunch Gyarados to death and deal a great deal of damage to Charizard. Aerodactyl, though, will probably outspeed and Rock Slide you into oblivion; skip it. The only way Typhlosion can deal any sort of significant damage to the Dragonites is by Defense Curl + Rollout, but this is a rather gimmicky strategy and thus not recommended; you will not be able to heal in between turns either, so just avoid it altogether.
From this point onwards, you can fight the gyms in any order, though you will need to retrieve the Machine Parts from the Cerulean City gym before you have access to the earlier portion of Kanto. Feel free to anticipate or postpone any battles as needed.
Gym #9 - Brock (Pewter City, Rock-type): ThunderPunch can dispose of Omastar and Kabutops, but that's about it. The rest is not something Typhlosion should fight; even Earthquake might not OHKO them with Typhlosion's average Attack and their high Defense. Don't try it.
Rival (Mt. Moon, optional): More of the same really. Alakazam might outspeed you, depending on your level, but Typhlosion should be able to take a hit, even if it does. Gengar should not outspeed.
Gym #10 - Misty (Cerulean City, Water-type): ThunderPunch can work against anything but Quagsire, but fighting in this gym is probably not a good idea. Water coverage is everywhere to be found, and Misty's Pokémon can probably take a hit even with the level difference. Avoid if you can, or use Sunny Day to increase your chances if you really must.
Gym #11 - Lt. Surge (Vermilion City, Electric-type): The Electrodes can explode. Watch out unless you're overleveled by some 10 levels or so, in which case Typhlosion should be fine if it has Earthquake. The rest goes down easily to that move or Fire Punch.
Gym #12 - Erika (Celadon City, Grass-type): LOL Erika, meet Fire Punch.
Gym #13 - Janine (Fuchsia City, Poison-type): ThunderPunch works really well against Crobat. The rest can be handled by Earthquake or, for the bugs, Fire Punch. They're all very underleveled, so chances are this fight will be a breeze.
Gym #14 - Sabrina (Saffron City, Psychic-type): Typhlosion's good special bulk makes it a capable fighter. You can opt for Fire Punch or your strongest physical move, which is probably Earthquake. Watch out though, Alakazam is still very strong; keep your health high while you fight it.
Gym #15 - Blaine (Seafoam Islands, Fire-type): While Magcargo does have Rock Slide, you have Earthquake and its Attack is ridiculous. Likewise, Earthquake can dispose of Magmar and Rapidash. This fight should be pretty easy.
Gym #16 - Blue (Viridian City): ThunderPunch is a godsend against Pidgeot and Gyarados. Alakazam is fine if you could take on Sabrina's, but remember that it hits hard. One Fire Punch and Exeggutor will be good as gone... and Earthquake disposes of Arcanine. Rhydon is the only one to avoid, it will NEVER be OHKOed by anything Typhlosion has.
Rival (Indigo Plateau, optional): Crobat will outspeed and Alakazam may or may not, depending on your level once again, but overall same fight as always. Just stay away from Feraligatr.
Red (Mt. Silver): The only two 'mons that you for sure can defeat are Pikachu and Venusaur, which are his two weakest links for sure. You should be able to beat the Charizard by using ThunderPunch, too. Blastoise, Espeon and Snorlax are better left for your teammates, though.
Before its final evolution, Cyndaquil has a pretty shallow movepool. It starts with Tackle and Leer, and gets SmokeScreen at level 6, which is only mildly useful. Ember only comes at level 12, likely too late even for the Sprout Tower challenge, though it is useful afterwards. Then, it gets Quick Attack at level 21 as Quilava, and by that point, not only you will already have Headbutt, you will also be levelled enough to not need a base-40 Normal move anyway. Flame Wheel comes at level 31, at long last. As Typhlosion, not much gets better; you have Swift at 45, way beyond the point of usefulness, and the very far off Flamethrower at 60, which is probably only on time for the Red fight (if you get that far).
TM-wise, Dig is an alright coverage move for Cyndaquil and Quilava, but whatever you do, DON'T use Dig against a Geodude because it will then be able to deal DOUBLE damage with Magnitude or worse even, Earthquake. Defense Curl and Rollout can be a decent combo, but just be aware that you will leave yourself wide open to an attack if you cannot KO the opposing mon quickly enough, and with Typhlosion's movepool it shouldn't be needed. Once you finally evolve into a glorious Typhlosion at level 36, the floodgates open. You can immediately teach it Fire Punch and ThunderPunch, as well as Earthquake just in time for the Elite Four. Cyndaquil also has access to the somewhat useful out-of-battle move in Headbutt, which also fills in an okay niche until Strength and other, better moves are available. As a filler move, Sunny Day is not bad; it doesn't come with SolarBeam, but it can protect Typhlosion against Water moves somewhat, and increase the power of its STAB.
Recommended moveset: Fire Punch / Flamethrower, ThunderPunch, Earthquake, Sunny Day / Headbutt / Strength
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.
Cyndaquil Line Ratings
At what point in the game should I be evolved? Take advantage of Cyndaquil's quick evolution and be a Quilava by the second gym (Bugsy). Typhlosion has a much easier time with gyms 5-7 than Quilava does, so it is suggested that you battle all non-mandatory trainers first and then battle Pryce, whom Typhlosion can easily beat with Fire Punch + ThunderPunch.
How good is the Cyndaquil line in a Nuzlocke? The Cyndaquil line is very good. It has good matchups against pretty much every single gym and Elite Four member, it has a great Fire+Electric attacking combination once it's fully evolved, and it has the speed and power to sweep entire teams with ease, a claim to fame that not many other Pokémon in Johto can rival. It is also available very early, is 100% obtainable, and has decent defensive stats for the rare occasions on which it doesn't score a OHKO. Definitely one of the best Pokémon you can get in GSC.
Weaknesses: Ground, Rock, Water
Resistances: Bug, Steel, Fire, Grass, Ice
Neutralities: Normal, Fighting, Flying, Poison, Ghost, Electric, Psychic, Dragon, Dark