Marvel Avengers Battle for Earth

Okay, this is the first "duo" achievement your going to have to do some work for. Wolverine is unlocked from the beginning of the game, but Phoenix is not. In order to earn Phoenix, you have to beat the first 4 battles of the "Manhattan" level in the Campaign. Once you do, she will unlock.

Now, simply go into versus, select these two, pound your non-existent opponent, and the achievement is yours!