BlazBlue: Continuum Shift

In order to pick the unlimited version of any character you must usually beat score attack, however it is automatically unlocked for the DLC characters. You can select them using LT in the character select screen while hovering over them, the unlimited text at the bottom of the screen should light up. If it does not, chances are you changed your control scheme and need to use default for this to work. Otherwise, figure out whatever you have it set to now and pressing that works all the same.

This achievement is called "With Our Powers Combined!" You get this for playing as Unlimited Platinum the Trinity. So just select the unlimited version of Platinum and you get the achievement. Simple as that.