This is a build that cycles around Hisuian Goodra (special), which has few weaknesses and high defensive stats, and Hippowdon (physical), repeatedly pressuring with Stealth Rock, Yawn, and Dragon Tail to get the ace, Mega Blastoise, through. As support, it adopted an unhurried Hippowdon and a Mega Meganium that is strong against physical Water-type attacks, a Toxic Spikes-setting Toxapex that checks Primarina and Meowscarada, and a Choice Scarf Gholdengo that cuts off consistent Close Combat from Mega Honchkrow.
The key winning route is that once Goodra has chipped away at Primarina in advance, Blastoise becomes selectable against almost any opponent, and the aim is to bait out the opponent's response on the team preview screen to create a favorable matchup.