The axis is Mega Gyarados, which can beat almost any opponent once it gets going, paired with a special ace (Quiver Dance set) to handle the Grass/Fairy types and Umbreon-likes that trouble Gyarados, and a Mimikyu that is strong against Garchomp and Marine Freeze. As leads, the team runs a Grass-type strong against stall/wall setups and a Garchomp in charge of Stealth Rock, rounded out with one more helper for type-matchup coverage.
The default selection is one of the two leads (the Grass-type or Garchomp) plus Gyarados, along with either the Quiver Dance ace or Mimikyu. The key piloting point is to hold off on Mega Evolving Gyarados for as long as possible, so it keeps applying type-switch selection pressure on the opponent. The last helper contributes little in actual play, but was adopted to bait the opponent's non-Mega picks and suppress the appearance of Marine Freeze and Staraptor-line Pokémon.