Here's an offensive core I've been using to support a Mega Gyarados sweep. I think Greninja's outlived its purpose and I'd like advice on how to improve it on either side.
Rotom (Rotom-H) @ Choice Scarf
Trait: Levitate
EVs: 252 SAtk / 4 SDef / 252 Spd
Modest Nature (+SAtk, -Atk)
- Volt Switch
- Will-O-Wisp
- Thunder Wave
- Overheat
This guy is a lovely little anti-meta lead. It catches all of the Greninja leads off-guard and OHKOs them with Volt Switch. It one-shots Ferrothorn and non-scarf Excadrill. Klefki leads die without accomplishing much. You can burn TTar and Mamo leads as they set up Stealth Rock, because both of those pokes when burned become set up fodder for Gyarados. If there's a fast Mega that's too threatening, you can shoot off a T-Wave and neutralize the threat. But, of course, Volt Switch is the #1 option for strong, fast damage.
Gyarados (M) @ Gyaradosite
Trait: Moxie
EVs: 40 HP / 252 Atk / 216 Spd
Adamant Nature (+Atk, -SAtk)
- Dragon Dance
- Ice Fang
- Waterfall
- Earthquake
At the center we have (my favorite poke of all time) the sweeper of the core that the other two are supporting by zipping around and weakening/eliminating counters. Rotom-H also sponges T-Waves and WoWs. As for weaknesses, what weaknesses? T-bolt incoming, just hit the Mega Evo button, tank the hit and DD. With the near-perfect coverage, it's very very easy to DD and get at least one Moxie boost before going Mega. He doesn't really switch out. Mold Breaker EQ generally seals the deal.
Greninja (M) @ Life Orb
Trait: Protean
EVs: 88 Atk / 168 SAtk / 252 Spd
Naive Nature (+Spd, -SDef)
- Hydro Pump
- Ice Beam
- Shadow Sneak
- U-turn
And on the other side, we've got the second part of the VoltTurn combo to wreak havoc. It's really all about Protean Ice Beam, taking out those Trevenants, Gourgeists, and all the dragons that Gyarados doesn't always want to face. Full special attack with Dark Pulse works too, but to be honest that Shadow Sneak has a shocking amount of utility especially in a core that otherwise lacks priority. He OHKOs so, so many threats and outspeeds almost the entire unboosted meta; he nails bulky Grass-types for Gyarados and always maintains offensive momentum.
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It needs a spinner/defogger, for sure, but otherwise Rotom-H and Greninja work wonders in opening up a sweep for Mega Gyarados. The only pokes that are notably difficult to deal with are Scarf Genesect and Mega Venusaur (even Mega Blastoise gets worn down pretty easily with the VoltTurn). If Rotom-H cripples a key threat like TTar, it makes Gyarados' set up even easier.
Anyways, let me know if there are better options with Gyarados! I've always wanted to try Gyarados + Lucario but haven't figured out how to work it yet.