The thing that stands out to me is that the monster's shoulders don't seem to follow the line of movement of its arm. The left shoulder should be coming forward a bit, since its arm is coming forward. Otherwise, I suspect it would pull a muscle trying to do that.
This is going in my notebook titled "Things I Didn't Know about Surface Dwellers."
The person ducking seems to have legs that are too long, or a smaller upper body, as the foreshortening seems a bit over-exaggerated to my eyes. Beyond that pretty simple fix, I think the wall needs a better impact effect where the fist hits, as someone else mentioned.
The speed lines over the guy's head made me feel like he wasn't directly under the arm. To me it looks like they're in front of the arm, so his head is too and he doesn't really need to duck.