Cliff is one of three Team GO leaders in Pokémon Go. To beat him, you’ll have to battle Pokémon like Gallade, Aerodactyl, and Dusknoir.
Gallade is a Psychic and Fighting-type Pokémon, which means it will be weak against Fairy and Flying-type moves. But it will also suffer greater damage from Ghost-type attacks.
It has a chance of being the second Pokémon you’ll face when battling against Cliff, so make sure you bring Pokémon such as Gengar with Lick and Shadow Ball to quickly and effectively faint Gallade.
Other Pokémon you can use are Gardevoir, Togekiss, Ivetal, Moltres, Mewtwo, Charizard, or Pidgeot, considering the high effective damage they can deal to Cliff’s Gallade. But any Pokémon that knows Fairy, Flying, or Ghost-type moves will give you an advantage.
This is also valid for when Gallade is featured in raids. But when it comes to Cliff’s battle, you’ll have to keep in mind his next Pokémon in the lineup when choosing your party.
The first Pokémon he’ll send is Aerodactyl, while the second Pokémon can be Gallade, Slowking, or Cradily. After you beat one of those, the third Pokémon can be Tyranitar, Dusknoir, or Mamoswine.