Details: One of the fourteen Special Trainers you will encounter at The Battle Tree after completing certain winning streaks or as a partner in Multi Battle Mode is Colress. Not only did he assist you in defeating Necrozma, but Colress also was key in stopping Team Rainbow Rocket during the RR Episode. He has a mix of Pokemon he uses.
Possible Pokemon:
Muk (Air Ballon): Minimize, Acid Spray, Rest, Snarl
Muk (Liechi Berry): Cruch, Belch, Shadow Sneak, Stone Edge
Electrode (Air Balloon): Thunderbolt, Mirror Coat, Thief, Protect
Electrode (Damp Rock): Rain Dance, Thunder, Taunt, Light Screen
Porygon-2 (Eviolite): Tri Attack, Thunderbolt, Psychic, Ice Beam
Porygon-2 (Eviolite): Tri Attack, Thunder Wave, Shadow Ball, Ice Beam
Metagross (Occa Berry): Earthquake, Meteor Mash, Bullet Punch, Explosion
Magnezone (Bright Powder): Charge Beam, Flash Cannon, Magnet Rise, Thunder Wave
Magnezone (Assault Vest): Thunderbolt, Flash Cannon, Tri Attack, Volt Switch
Porygon-Z (Expert Belt): Psychic, Ice Beam, Signal Beam, Charge Beam
Porygon-Z (Normalium Z): Tri Attack, Thunderbolt, Shadow Ball, Recover
Klinklang (Steelium Z): Shift Gear, Gear Grind, Volt Switch, Giga Impact
Klinklang (Leftovers): Flash Cannon, Thunderbolt, Protect, Toxic