Supergirl can shoot Kryptonian Lasers by pressing the
button. This move is governed by a meter next to her super bar, and will need waiting time to recharge.
To earn this trophy, go into either Versus or Single Fight (Very Easy difficulty of course) and choose Supergirl as your character, then Superman as your opponent. Simply spam this move until either you defeat the man of steel, or until you count 20 connected moves (i.e., they can't miss or be blocked) and the trophy will unlock.