Butterfree 21

Your opponent switches the Defending Pokémon with 1 of his or her Benched Pokémon, if any. (Do the damage before switching the Pokémon.) Damage: 20

Remove a number of damage counters from Butterfree equal of half the damage done to the Defending Pokémon (after applying Weakness and Resistance) (rounded up to the nearest 10). If Butterfree has fewer damage counters that that, remove all of them. Damage: 40