Cliff is one of the Team GO Rocket Leaders that you can battle on Pokémon GO after collecting Mysterious Components.
Cliff's 1st Pokémon: Pinsir
Cliff's first Pokémon has to be Pinsir. Here are the best Pokémon to beat him easily:

Fire Fang and Overheat

Hex and Overheat

Wing Attack and Overheat
Cliff's 2nd Pokémon
Cliff's second Pokémon is randomly chosen between Omastar, Electivire, or Marowak. Here are the best Pokémon to easily defeat each of them:
To defeat Omastar

Vine Whip and Frenzy Plant

Bullet Seed and Frenzy Plant

Razor Leaf and Grass Knot
To defeat Electivire

Mud Shot and Earthquake

Mud Shot and Earthquake

Mud Shot and Earthquake
To defeat Marowak

Mud Shot and Outrage

Mud Shot and Earthquake or Hydro Cannon

Waterfall and Hydro Pump
Cliff's 3rd Pokémon
Cliff's last Pokémon is randomly chosen between Swampert, Torterra or Tyranitar. Here are the best Pokémon to easily defeat each of them:
To defeat Swampert

Vine Whip and Solar Beam

Fury Cutter and Leaf Blade

Quick Attack and Leaf Blade
To defeat Torterra

If Torterra has the move Razor Leaf or Bite.
Ice Shard and Icy Wind

If Torterra has the move Razor Leaf.
Dragon Breath and Draco Meteor

If Torterra has the move Razor Leaf.
Extrasensory and Future Sight

If Torterra has the move Bite.
Snarl and Sky Attack

If Torterra has the move Bite.
Powder Snow and Avalanche
To defeat Tyranitar

Counter and Focus Blast

Counter and Dynamic Punch

Counter and Close Combat
The reward for defeating Cliff is a Shadow Pinsir. It can be shiny.

Learn about the different Pokémon Sierra has when you play against her and how to beat them easily on Pokémon GO.