This trophy is titled "Restraining Order" and it's for using barrier burst 30 times. Barrier burst is when you press all four buttons at the same time (X, Circle, Square and Triangle) and it launches the opponent in the air. You are allowed one of these per round, you can tell you haven't used it when the red BB near your health bar is still present.
The quickest way to grind these out would be to start in either Versus or even Arcade. Upon the start of each match, use Barrier Burst and then play the round out as you would. Set the round count to 5 and the timer to 30 in case the battle takes a bit longer than expected. The trophy says to use the barrier burst but I would make sure it connects just to play it safe.