Gyarados-Mega @ Gyaradosite
Ability: Intimidate
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Dragon Dance
- Crunch
- Waterfall
- Substitute/Earthquake/Ice Fang

Tapu Bulu @ Fightinium Z / Choice Band
Ability: Grassy Surge
EVs: 252 HP / 252 Atk / 4 Def
Brave Nature
IVs: 0 Spe
- Swords Dance
- Horn Leech / Wood Hammer
- Superpower
- Megahorn / Stone Edge / Zen Headbutt

Marowak-Alola  @ Thick Club
Ability: Lightning Rod
EVs: 252 Atk / 4 SpD / 252 Spe or 184 HP / 252 Atk / 4 Def / 68 Spe
Adamant Nature
- Shadow Bone
- Fire Punch
- Bonemerang / Stealth Rock
- Will-o-Wisp / Stealth Rock

The core possesses incredible type-sinergy, and it's mainly centered around providing support for Mgyara to sweep. MG gets completely destroyed by Pheromosa and Koko, other than that grass and fighting types trouble him a lot. When they are gone, its absurd natural bulk and good STABs can easily provide a sweep.
A-marowak with Lightningrod is a great switch-in to Koko and a very good switch-in to pheromosa, resisting all of its common attacks, needless to say he absolutely destroys grass types and some walls like skarmory that can trouble MG. In return, most of the Dark-typed things that stop marowak from wreaking havoc can't really do shit to Mega gyarados.
Finally, defensive tapu bulu was put there to support the former two guys, weakening earth moves for A-Maro, providing good passive recovery to both of them thanks to leech seed and grassy terrain; he also does an outstanding job at stopping keldeo and most bulky grounds, which are pretty much the only threats to MG that marowak can't check.