It's a Triple Battle team.
Gyarados w/ Gyaradosite (Dragon Dance, Outrage, Bite, Waterfall)
Amoongus w/ Black Sludge (Protect, Rage Powder, Spore, Giga Drain)
Azelf w/ Light Clay (Taunt, Psychic, Light Screen, Reflect)
Greninja w/ Expert Belt (Scald, Dark Pulse, Grass Knot, Ice Beam)
Darmanitan w/ Life Orb (Protect, Superpower, Flare Blitz, Rock Slide)
Butterfree w/ Choice Scarf (Safeguard, Sleep Powder, Bug Buzz, Rage Powder)
My lead is Gyarados - Amoongus - Azelf. The strategy is to lead with Gyarados' Intimdate and Azelf's dual screens to establish a strong defensive presence. Amoongus Rage Powders to draw attacks/statuses away from Gyarados so that I can Dragon Dance a few times as needed. From there, it's pretty elementary; in most cases, Gyarados sweeps with Greninja and Darmanitan there to provide additional attacking support as needed. If Amoongus gets KO'ed, Butterfree comes in and starts putting things to sleep.
It's a surprisingly simple team, really. In most battles, I only lose 2 pokemon the whole battle (Amoongus usually falls due to all the attacks it takes and how many weaknesses it has, and by the end of a battle, I often lose either Azelf or Butterfree, too).
As of last night, I'm at around 87 or 88 wins.