Many characters have SP moves that they can only use while Awakened. These Sp moves can also be improved upon by using two action buttons instead of one, thus using more meter in order to unleash a stronger attack. Here's how I unlocked this one...
I picked Yu and started up Arcade Mode on safety. I let the opponent whittle my health down until I awakened (35% health, your characters face appears on the screen and your meter gets a boost). Then I did a double quarter-circle back + Y and B and landed the SP attack, the acheivement unlocked.
In short, the two important criteria are making sure you are Awakened and using Y+B to get the move off. Many characters have these kind of moves that can only be used when they are Awakened, it should say this in the description. This is just the one I used for the achievement.