by nekobat
The top card of the Super-Villain stack is the card on top of it, i.e. the one that's face-up.You always resolve a cards effect completely. J'onn specifically says to "return it to the top of the stack" after you play it. Note that it doesn't say return it at the end of your turn. Therefore, you can defeat the Super-Villain the same turn you play it with J'onn.
(CZE Game Tester)