A screens-support build where Grimmsnarl sets both Reflect and Light Screen simultaneously to reinforce the whole team's bulk. Under that shield, Volcarona and Mega Gyarados look for a stacking sweep with Quiver Dance and Dragon Dance respectively. While the screens hold, it's hard for even strong hits to bring down the sweepers.
Grimmsnarl (Light Clay, Prankster): Reflect / Light Screen / Parting Shot / Foul Play. Light Clay extends screen duration, greatly raising the whole team's survivability as the support.
Volcarona (Sitrus Berry, Flame Body): Fiery Dance / Will-O-Wisp / Quiver Dance / Morning Sun. Quiver Dance raises both Sp. Atk and Speed together to sweep safely under the screens.
Mega Gyarados (Intimidate): Crunch / Earthquake / Substitute / Dragon Dance. Settles in safely with Substitute after Intimidate, then accelerates with Dragon Dance as the closer.
Garchomp (Lum Berry, Rough Skin): Earthquake / Scale Shot / Rock Slide / Swords Dance. Endures one more hit thanks to Lum Berry, then raises closing power with Swords Dance as the second sweeper.
Mega Floette (Flower Veil): Moonblast / Psychic / Draining Kiss / Calm Mind. Handles the late-game special line with dual Fairy/Psychic firepower boosted by Calm Mind.
Gholdengo (Leftovers, Good as Gold): Make It Rain / Shadow Ball / Nasty Plot / Recover. Combines Nasty Plot with Recover to stall tenaciously behind the screens.