Amulet of Anubis is the Supermove of Dr. Fate, which is performed by pressing the L2+R2 buttons once the super meter is filled. Performing special moves (which can be looked up in the pause menu) fills it quicker.
Easiest way to do this is either by loading up a versus match with a second controller or single fight on Very Easy difficulty. Just be aware, on the latter they may block it. You want to perform this when the opponents health is down to around a quarter of red or 25%, because if you do it too low then it'll just KO them before performing the move.