The Beast Below is Aquaman's supermove. As with all other supermoves, you must simply press
+
with four filled super meter bars to perform it. As Aquaman, you must defeat any opponent with this move.
So, load up either two-controller versus or single fight on very easy, pick Aquaman and then any opponent and use special moves (found in the pause menu) to fill up the meter quickly. Then, once your opponent is down to around 25% of health, perform the supermove and the trophy should pop after the match.