BlazBlue: Continuum Shift

"Three Wolf Moon
Used the Unlimited version of Valkenhayn."

Do this the exact same way as you did Makoto and/or Platinum.

Go into any mode that allows you to play as Valkenhayn (I chose Versus) and select Unlimited Mode before choosing him, located at the bottom left or right (depending on which player you are) of the screen, in the same area of the screen where you pick Beginner Mode. As soon as you enter the fight the achievement will pop.

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.