Create an ability, make it realistic or broken, do whatever you want:

Sabatoge - HP affected moves on the opponent pokemon have the opposite effect (so something like recover would drain HP and something like double edge would heal HP)

Loose Thorns - If this pokemon is hit with a move that makes contact then a layer of spikes is spread around the opponent's feet

Vengeance - If this pokemon replaces a pokemon that was knocked out the previous turn than it receives a one stage boost to attack and special attack