Launch a super effective Shadow Ball!
From Create Your Own Story
It would seem like the best choice. Your little Eevee whips out a big ball of blackness and sends the Mew off the wall of the lab. Unfortunately, this Mew will not go down easily. It retaliates with Psychic, smacking little Eevee to the floor. Eevee is down and you are out of usable Pokémon. It would seem beating it up wasn't the best strategy... Try again?