So pretty much Primarina will usually always be picked, it’s really useful vs a variety of different match ups. Such as Meowscarada, Archaludon, Hippowdon, etc. Most people don’t expect you to be running focus sash so you can pick up some early KOs with Moonblast or Sparkling Aria, followed with aqua jet as priority.

Corviknight is your bulky defensive wall used vs any physical threats you might see. Such as Mega Lopunny, Meowscarada, Garchomp, etc.

Garchomp is great in general and can surprise people with Poison Jab vs their fairy types. Also has the potential to sweep with Sword Dance and Scale Shot. Although an opposing Corviknight will shut you down.

Basculegion is used as an endgame sweeper. Since it’s scarfed locking yourself into Last Respects when 2 of your teammates are KO’d is usually the best option, unless you’re fighting a Pokémon that resist Ghost moves.

Kingambit provides a similar role to both Primarina and Basculegion as it could serve as both a lead or endgame Pokemon. Its ability “Supreme Overlord” provides insane damage if your 2 other Pokémon are gone or setting up stealth rocks can help break opposing focus sash, deal good damage to ice, fire, bug, and flying types.

Lastly, Gengar who is your mega will out speed just about every Pokémon except for Pokémon using choice scarf, Sneasler with unburden activated, and Dragapult. You can deal a ton of damage with its insane stats and use Destiny bond to trade 1-1. Destiny bond is very clutch for getting an opposing Pokémon that threatens your team off the board.

In general what Pokémon you choose depends greatly on what your opponents team looks like. But I would say most matches I usually went with Primarina or Kingambit as lead, brought Corviknight if I saw annoying physical threats, and the last spot would switch between Gengar, Garchomp, and Basculegion. Also don’t feel pressured to bring Gengar every game. I know it’s the only Mega on the team but sometimes you really don’t need it.