Zelda II: The Adventure of Link/Rebonack
Here's an image by unwary demonstrating how to line up for and execute the easy kill on Rebonack.
Note that if he wanders right when you're on top of him, you may fall off of him and have to fight him the old, slow, dangerous way.
If he wanders left, however, and goes off the screen, and you go to the floor, have no fear. Just jump back onto him when he comes back on screen, and you can get right back to bouncing.
But beware! If he's too far off screen when he dies, the key may not drop, and you'll be stuck and have to reset.