Exile can deal with that, but black mana rarely exiles creatures, so black Commander decks can use Toxic Deluge instead
Make sure youre drawing and attaching Energy wisely so that if one Mewtwo goes down, you have the resources to power up another
If a player creates a token that's a copy of Xantcha, the player who creates the token is its owner, not the player under whose control it enters the battlefield
(8/25/2017) If a creature has been dealt damage, that damage remains marked on it until the cleanup step
Outside red, every other colour is important
You get to trigger its explosively recursive ability whenever it mutates, and you can use cards like Buried Alive or Final Parting to tutor creatures into your graveyard that later can enter the battlefield for free