Treecko is found in both games as a starter, received from Professor Birch in Littleroot Town. It will be marked as a Route 101 Pokémon, however.
Though still widely regarded as the hardest Hoenn starter to use in a Nuzlocke, the Treecko line received some very welcome gifts in ORAS that a Generation III grassy gecko could never have dreamed of. These include, but are not limited to, early Grass-type moves other than Absorb, moves such as Focus Blast, Grass Pledge, and Leaf Storm, and a nifty new Mega Evolution. Despite all these boosts, though, the Treecko line maintains many of the flaws it had in RSE. Physical frailty, not overwhelming offensive stats, and a typing that doesn't do so well in a few certain stages of the game (particularly the mid-game) all work against the original Smugleaf and its evolutionary relatives. In the hands of a knowledgeable Nuzlocker, however, Treecko and its evolutions can put in some fantastic work in the tropical region of 7.8/10 too much water. (Mega) Sceptile turns out to be a lot more of a late game Pokemon than Blaziken due to all the watery segments, and the gap between Blaziken and Sceptile is much smaller than it was before.
Rival (Route 103): Just grind to around Level 7 or 8. If you don't feel like being overleveled, just remember that it's generally better to use Leer once before letting loose with Pound.
Gym #1 - Roxanne (Rustboro City, Rock-type): If Treecko is at or above Level 13, this battle will be a cinch. Mega Drain comes at that level, and it has a 100% chance of bringing Geodude down to 1 HP. The best strategy to use against the Sturdy little bugger is to start off with either Absorb or Mega Drain. Its Sturdy will activate, allowing it to get an attack - probably a Rock Tomb - off. She'll heal, so it's best to use Pound to break its Sturdy. Then, follow up with an Absorb or Mega Drain to KO it and heal yourself in the process. Nosepass is slightly trickier, as it doesn't have a 4x weakness to Grass, but it should go down to anywhere from two to four Mega Drains. Thankfully, its pitiful Attack stat means its Rock Tombs won't do much. You should be able to heal off most, if not all, of the damage it does to you by using Mega Drain.
Gym #2 - Brawly (Dewford Town, Fighting-type): It's not a very good idea to rely on Treecko/Grovyle for this battle. Brawly's Pokémon have high Attack stats, and their Fighting STABs will hit Treecko's/Grovyle's low Defense rather hard. Though Treecko/Grovyle can easily outspeed all of Brawly's Pokémon, it probably won't even be able to score a 2HKO on either of them due to the surprising overall bulk of Machop and Makuhita. If you can, let a bird or bug handle this one.
Rival (Route 110): Grovyle should easily be able to one-shot the Wailmer, but Shroomish is bulky and will resist your STAB moves. Grovyle can wall Shroomish rather well, but it can't do much to it in return. I shouldn't even have to say this, but don't use Grovyle against Combusken.
Wally (Mauville City): Wally's only Pokémon, Ralts, can at best use Disarming Voice to damage Grovyle, and it doesn't do much. This battle should be a cakewalk.
Gym #3 - Wattson (Mauville City, Electric-type): With Low Sweep and Grass Pledge in hand, Grovyle can put in some good work against Wattson. Low Sweep will most likely bring Magnemite down to 1 HP (it has Sturdy); make sure Grovyle's holding a Cheri Berry in order to make the Thunder Wave that it'll probably use a non-issue. Voltorb is frail and will likely go down to a Grass Pledge if you're at a decent level (anywhere from 21-25). Magneton will probably be able to take two or more Low Sweeps, but it can't do much back to you: Supersonic is inaccurate and reliant upon hax, Magnet Bomb is weak, and Volt Switch is resisted.
Tabitha/Shelly (Meteor Falls, tag battle with rival): If you're playing Omega Ruby, switch the hell out. Numel has Flame Burst and Lava Plume, both of which are very powerful and will most likely 2HKO or OHKO Grovyle. If you're playing Alpha Sapphire, however, this battle will be relatively easy; just make sure to switch out once Grimer comes into play, though, since Grovyle can't reliably beat it.
Tabitha/Shelly (Mt. Chimney): In Alpha Sapphire, Leaf Blade or Grass Pledge should be more than enough to deal with Shelly's Carvanha, but not her Grimer. However, in Omega Ruby, there is not much to do for Grovyle here.
Maxie/Archie (Mt. Chimney): Omega Ruby? Mightyena is the only thing you should be fighting, period. Golbat and Camerupt both wreck you in a rather major way. Mightyena, on the other hand, can be annoying with Swagger and Roar, but it isn't too dangerous. In Alpha Sapphire, Grovyle should be fine with taking on Sharpedo. However, don't switch Grovyle in on one of its attacks: it has Ice Fang, which will almost surely OHKO Grovyle. If you're able to get a free switch-in, though, Grovyle should be able to outspeed and OHKO with Grass Pledge/Giga Drain.
Gym #4 - Flannery (Lavaridge Town, Fire-type): You might as well put Grovyle in the PC before you walk into this gym. It's not going to be able to help unless you taught it Dig... in which case it might help a little bit before it dies due to a careless mistake. Trying to use Dig Grovyle to handle Flannery is like trying to use a Fire-type to handle Lorelei. It doesn't work, so don't do it.
Gym #5 - Norman (Petalburg City, Normal-type): Using Grovyle's Low Sweep in this battle may seem appealing, but let me assure you, it's not a good idea. All three of his Pokémon have Retaliate, which will hurt especially bad when coming off of one of the Slaking's massive base 160 Attack stat. Combine that with the titanic bulk that both of his Slakings have, and you've got a battle that's going to be very, very dangerous for your physically frail starter.
Courtney/Matt (Southern Island, tag battle with Steven): Your Grovyle/Sceptile should sit this battle out. While it can probably beat Sharpedo with a Leaf Blade (if you're playing Alpha Sapphire), it'll get hit hard by super-effective Poison-type moves from the shark's battle partner. And if you're playing Omega Ruby... don't even think about it. Koffing isn't fun to fight with Grovyle/Sceptile, but Camerupt will be a nightmare.
Tabitha/Shelly (Weather Institute): Going against Shelly? Go for it. Sceptile (which you should have by now) will easily be able to outspeed and OHKO her Sharpedo. Going against Tabitha's Camerupt, however, is and always will be a bad idea, even if your Sceptile has Dig.
Rival (Route 119): As with before, Wailmer will be very easy to take out. Shroomish may be annoying to take down, but as Sceptile walls it pretty well, it shouldn't prove to be too tough of an adversary. This time, a Sceptile at a reasonable level should be able to one-shot Combusken with Dig. Its best Fire move is Flame Charge, which, despite being super-effective and getting the STAB boost, is not very strong.
Gym #6 - Winona (Fortree City, Flying-type): Leave Sceptile out of this one. If you really need something to take on her Altaria, though, you can use Sceptile's/Mega Sceptile's Dual Chop. However, if you do decide to Mega Evolve, remember to take into account that Winona's Altaria is carrying Dragon Breath. Also remember that Cotton Guard will make it nearly impenetrable on the physical side, so if it does use that, just switch out to a special attacker, preferably one not weak to Earthquake or Dragon Breath.
Courtney/Matt (Mt. Pyre): Let your Water-type handle Courtney if you're playing Omega Ruby. If you're playing Alpha Sapphire, though, Matt's Sharpedo should be easy pickings for Sceptile.
Rival (Lilycove City): Your rival now leads with a Swellow, so make sure Sceptile isn't at the front of your party when you start this battle. Wailord is still an easy target for Sceptile, but Blaziken and Breloom are too powerful for the frail lizard to handle. Use a Flying-type against them, if possible.
Courtney/Matt (Lilycove Hideout): For the first time in forever, Courtney's Camerupt isn't as dangerous to Sceptile as Sceptile is to it. It lacks Fire-type moves, so feel free to let loose with super-effective Ground-type moves or powerful STAB Grass moves. If you're facing Matt, well, you should know by now that Sceptile absolutely demolishes his Sharpedo.
Gym #7 - Tate and Liza (Mossdeep City, Psychic-type): Sceptile does very well in this battle, especially if you're allowing it to Mega Evolve. A Grass STAB should be able to take out Solrock in one shot (Grass Pledge, Giga Drain, or Leaf Storm all work well for this purpose), and Lunatone should go down to one or two Grass moves. If you don't want to let Lunatone set up a Light Screen, though, I'd suggest gunning for it before trying to take down Solrock.
Maxie/Archie (Seafloor Cavern): Sceptile can take on Maxie's Mightyena and Mega Camerupt without any significant issues. His Mega Camerupt lacks Fire-type moves, making it relatively safe for non-Mega Sceptile to handle. Avoid Weezing and Crobat at all costs, though. A similar strategy should be exercised if you're playing Alpha Sapphire and going up against Archie: take out his Mightyena and Mega Sharpedo, but keep well away from Muk and Crobat.
Primal Groudon (Cave of Origin, Omega Ruby only): Its Lava Plume will almost surely score a OHKO whether you decide to Mega Evolve or not. Mega Sceptile, on the other hand, will probably not even be able to 2HKO with its Grass STABs. Keep your starter well away from this primal behemoth.
Primal Kyogre (Cave of Origin, Alpha Sapphire only): It has Ice Beam. Regular Sceptile will get decimated by it due to it lacking the power to be able to 2HKO Kyogre with Grass moves, while Mega Sceptile will probably not be able to OHKO with Leaf Storm and will subsequently get decimated by Ice Beam. Using Sceptile here might seem like a good idea, but using your starter here will almost certainly result in its death.
Gym #8 - Wallace (Sootopolis City, Water-type): Sceptile, Mega or not, makes this battle a cakewalk. Everything sans Sealeo and Milotic are complete pushovers, and even Sealeo should go down to a single Giga Drain or Grass Pledge. It's best to use Leaf Storm against Milotic, though, as it has insane Special Defense and a hard-hitting Ice Beam to retaliate with. Mega Sceptile should be able to OHKO Milotic with Leaf Storm, though, so don't worry too much about it, but switch out if you're unsure.
Wally (Victory Road): Altaria should be relatively easy for Mega Sceptile to beat with Dual Chop or Dragon Claw. Delcatty is extremely frail and should go down to a single Grass STAB. Magneton is annoying due to it having Sturdy, but Sceptile can easily defeat it with an Earthquake followed by a finishing move. Sceptile lacks the moves to do much to Roselia, but a Dragon-type move from Mega Sceptile should be able to score a clean 2HKO or OHKO. Wally's Mega Gallade, though, is a bit too strong for the physically frail Sceptile to handle; it's better to switch out against that offensive behemoth.
Elite Four Sidney (Ever Grande City, Dark-type): Sceptile's ability to learn strong Grass and Fighting moves really shines through in this battle. Focus Blast handles Cacturne and Shiftry, while Sharpedo, Absol, and Mightyena should be frail enough to fall to a Grass STAB or two. A Grass move from Mega Sceptile will likely OHKO several of Sidney's Pokemon if you're a few levels above (and you should be).
Elite Four Phoebe (Ever Grande City, Ghost-type): This isn't an awful matchup for Sceptile, but it isn't necessarily a good one, either. Though Sceptile has access to Night Slash, that move isn't particularly viable to keep on it in the long run. If you do choose to use Sceptile in this battle, though, just beware of Dusknoir's Ice Punch and Fire Punch.
Elite Four Glacia (Ever Grande City, Ice-type): If you're really, really desperate, Sceptile should be able to handle Walrein. Just remember that it carries Blizzard and probably won't go down to one Grass move, though. Chancing the Mega Evolution + Leaf Storm is even a pretty risky move, as though Leaf Storm will almost surely score the OHKO, it has a 10% chance of missing.
Elite Four Drake (Ever Grande City, Dragon-type): It's really better not to use Sceptile, Mega or not, here. Dragon Claw isn't exactly the strongest move ever, and Drake's Dragons are rather bulky. Altaria and the Flygons should be safe to take on, but only risk it if you're capable of Mega Evolution and willing to take a chance.
Champion Steven (Ever Grande City, Steel-type): Skarmory, Armaldo, and Mega Metagross will shut you down, but Focus Blast, Earthquake, and Grass STABs will allow you to go to town on the rest of Steven's team. Claydol should be easily one- or two-shotted by a Grass move, Aggron will go down to either a Focus Blast or an Earthquake (be aware that it has Sturdy, though), and Cradily should be 2HKOd by Focus Blast. Cradily does carry Sludge Bomb, though, so be careful. As implied before, don't use Sceptile against Skarmory, Armaldo, or Mega Metagross.
Matt/Courtney (Petalburg City, Postgame): For Courtney's Camerupt, it's ideal to go mega, as Earthquake is a safe 2HKO and Camerupt's only fire move, Eruption, only does 18 damage at most. Matt's Sharpedo is even more of a joke with a STAB Grass Type attack.
Zinnia (Granite Cave, Postgame): Tyrantrum easily kills Mega Sceptile and heavily damages regular Sceptile with Dragon Claw so make sure it stays out for that fight. Altaria easily gets 2HKO'd by Dragon Claw or Dual Chop, but Altaria 2HKOs Mega Sceptile in return. Salamence kills Mega Sceptile just as easily as Tyrantrum.
Courtney/Matt (Mossdeep Space Center, Postgame): Courtney's Camerupt as well as Matt's Sharpedo now mega evolve. Now an Eruption from Mega Camerupt will 3HKO Mega Sceptile instead of 9HKOing it at equal level, barring a critical hit. Mightyena is nothing worth fretting over at this point in the game, but the Weezing the grunt has will avoid any potential earthquakes Mega Sceptile may throw and can explode in its face, causing a lot of damage if not killing it. Matt, however, is a lot easier. Sharpedo will die to any grass type STAB you throw at it and Mightyena isn't exactly known for its bulk. The grunt in Alpha Sapphire has a Muk instead, which, while it can be hit by Earthquake, is only 2HKO'd by it and Mega Sceptile is taking a lot of damage from Gunk Shot, if not getting killed by it.
Wallace (Route 131, Postgame): His team is the same from Emerald; same strategy from Emerald.
Rayquaza (Sky Pillar, Postgame): Probably not a good idea to take him on with Sceptile if you wish to catch him without a Master Ball. Its moves will utterly destroy your green lizard friend without even trying. Safest bet is to use someone else like a Fairy type.
Zinnia (Sky Pillar, Postgame): There is a reason you can't start the fight without Rayquaza.
Deoxys (Space, Postgame): Probably a good idea to start the fight with Rayquaza.
Wally (Battle Resort, Postgame, 1st time): Same team as in victory road, but with higher level Pokemon. Also his Roselia is now a Roserade, but using the strategy from the last fight works here.
Wally (Battle Resort, Postgame, 2nd time): His team adds a Garchomp of all things, and the levels are higher. It would be a dumb idea for Mega Sceptile to fight him, like Tyrantrum for the same reasons.
As usual, Treecko starts off with Pound, Leer, and Absorb. Quick Attack comes at level 9, and while it outclasses Pound, it isn't entirely useful, as the Treecko line is incredibly fast as is. Mega Drain at level 13 is very nice, and it comes at just the right time: by this point, you should be just about to battle Roxanne, and this move works perfectly for that battle. It'll also be your main STAB up until around Mauville City. Should you choose to delay the evolution of Treecko, Giga Drain will come at level 21 and Energy Ball will come at 37; however, whether you choose to delay the evolution in order to get one of these moves is a personal choice. The former in particular is very useful because of it healing your Grass starter, increasing its longevity despite the low defenses. Grovyle gets Fury Cutter upon evolution and Pursuit soon after, neither of which are particularly notable. Leaf Blade, which comes at level 23, does happen to be a rather nice move for Grovyle, though. The next move of note comes upon evolution: Dual Chop, which is learned by Sceptile at level 36, doesn't seem all too great until you consider that it's one of two Dragon-type moves that Sceptile, and hence its Grass/Dragon Mega Evolution, can learn before you reach the Hall of Fame. X-Scissor comes at level 45, and is a nice move for dispatching opposing Grass-types, as well as pesky Psychic- and Dark-types. Leaf Storm comes at level 63, and while it's very powerful, it has a rather harsh drawback that you may not want to risk. Leaf Storm and Night Slash are both available via the Move Reminder (Sceptile only).
As for TMs... Dragon Claw is superior to Dual Chop due to the former's higher accuracy, so make that replacement once you find TM02 deep in the depths of Meteor Falls. Earthquake and Focus Blast are always essential coverage moves, though the awful accuracy of the latter may deter you from using it. Moves such as Dig, Bulldoze, and Low Sweep can be used as early-game substitutes. Rock Tomb and Rock Slide are interesting choices, as they allow you to hit Flying-types that Sceptile otherwise is rather helpless against. Aerial Ace and Acrobatics can be used, though the former is weak and the latter requires Sceptile to not be holding its Mega Stone (or anything else, for that matter) in order to be powerful, which is a bummer. The TM for X-Scissor can be used if you weren't able to accommodate it when Sceptile attempted to learn it via level-up. Swords Dance is a good option if you're running a physical set, but it comes at the cost of coverage. Moves such as Grass Knot and Nature Power can be used, but they're rather gimmicky. The final move worthy of mention is Grass Pledge, which a Mauville City tutor will be willing to teach to Treecko/Grovyle. It has 80 base power and receives STAB, so it'll be one of the strongest moves in your team's arsenal for quite a while. Its availability is the main reason why you may not want to delay Treecko's evolution in order to obtain Giga Drain.
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.
Treecko Line Ratings
What Nature do I want? Something that increases one of its attacking stats, preferably Special Attack. While decreasing Attack is annoying due to Sceptile numerous physical coverage options, you certainly shouldn't be disappointed if you get a Modest Nature.
At what point in the game should I be evolved? If you want, you can delay Treecko's evolution until Level 21, which is when it gets Giga Drain. However, despite Giga Drain's usefulness as a healing move, Grass Pledge comes at around the same time and is slightly more powerful, though it lacks the handy built-in recovery of Giga Drain. If you're allowing use of the Exp. Share, then you shouldn't have to worry about delaying the evolution, as Treecko won't really have to fight anyway; however, if you're disallowing the Exp. Share, then it might be best to simply evolve into Grovyle at Level 16 (which should be either slightly before or slightly after Brawly). If you're an overgrinder and a cautious player, you might have a Sceptile before Norman; if not, it's best to at least have evolved your Grovyle before you fight Winona.
How good is the Treecko line in a Nuzlocke? Very good, but not quite as versatile as Mudkip or as powerful as Torchic. Sceptile's niche is that it's very fast and has decent mixed attacking stats to back up its interesting movepool, which allows it to get the jump on almost every single Pokemon in the game, especially once Mega evolved. Just don't expect it to be OHKOing many things with physical coverage moves, watch out for its physical frailty, and make sure you know which TMs it wants, and you should be golden. Sceptile and its Mega are very much late game Pokemon, and can have usage in a lot of major battles during the Elite Four through sheer coverage alone, but Grovyle will struggle in some sections mid-game, but not that much due to getting great Grass moves.
Treecko, Grovyle, and Sceptile's type matchups:
Weaknesses: Flying, Bug, Poison, Fire, Ice
Resistances: Grass, Ground, Water, Electric
Neutralities: Normal, Fighting, Rock, Ghost, Steel, Psychic, Dark, Fairy, Dragon