Go into battle and place an ability you do not want on your Pokémon at the bottom by pressing Select. Go to the Item List and go down the to the seventh item from the top, press Select on it, and exit out. Go into a battle and press select on the bottom ability, and then attack the enemy and defeat it, without pressing select again. The Pokémon will then be L:100, but the ability you pressed select on will be gone! It's health bar may do some weird stuff, but just go to a PokéCenter and he'll be good as new.