Whenever he uses the move where he just punches straight (the one that knocks you back really far), dodge it so that you end up on the right side of his arm. His face will be low enough to hit.
You must kill him by getting the last hit on his face. It doesn't do much damage, so you'll want his health to be pretty low
(I've now noticed that a few more people have this achievement than defeating him, so I am not sure if my game just wasn't giving me the achievement until he killed him like this cuz it's buggy or if you actually have to kill him like this)