This achievement is for using Aquamans super to defeat an opponent. While using Aquaman as your character get your opponent's health down below 20% of the second meter. Store your burn meter up until you have all 4 meter portions. Then hit the left and right triggers at the same time.
Aquaman's super has him run in a straight line in the direction he is facing, so make sure your opponent is in front of you when you trigger the ability. Supers are blockable so you may want to play low level ai to force the situation.