Phantom Breaker

short version: fill the meter you see under the timer during the fight and hold if you see the bar flashing to release an critical-brust.

long version: go to the arcade mode and press start with your second controller to bring in another player. I used Cocoa for my main gamertag and Infinity for player 2. With both characters pretty close each other i started to press X with both controllers to use counter attacks - that way to bar under the timer fills pretty fast. However... feel free to use any other way to fill the bar. Just wait until the bar is full and flashes... now quickly hold the button and start the special attack - boom! achievement unlocked.