Butterfree

Whirlwind

20

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

Mega Drain

40

Remove a number of damage counters from Butterfree equal to 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 than that, remove all of them.