Seeing as this looks like a side scroller, I have a (personal preferenced) suggestion.  In my opinion, the hammer shouldn't cut into the platform on which he's standing. 
Anyway, here's how I might have done it:
  Excuse the hair/messy rotation/fact that the handle moves up and down as well as tilting.
Even if you disregard my first point, I still think the hammer shouldn't shift forwards an backwards as much as it should tilt back and forth a bit.  Its heavy, so he wouldn't move it a lot if he's just idle.

That's the neat thing about moves like these...or maybe just animation in general.  There are so many different ways to portray an action.  The most widely recognized example would probably be the walk cycle, which depending on how it is done can convey a very distinct character/personality.  But really anything has several different takes..that's what I like so much about forums, since you get to see what other people have for input. 
Hope this isn't too off topic.   

Yeah, the reason I didn't fix the outline is 'cause it was just an edit.  It wasn't supposed to be any sort of  final product, but more of a suggestion to show what weight can add to that type of attack. 
As for the axe outline, you still could clean it up a bit, something in this direction:

Here's my suggestion for a change on that axe attack.  The dust clouds and ground shaking at the end are just my little preference, ignore if you don't care for them.  Please ignore the white junk on the axe; it is there due to my craptastic rotation tool! 

Imo, the axe needs more weight.  He should have to swing it back further to get that much of a swing, even if he is super powerful...

Also, in yours he sort of jumps forward and then is suddenly back a few feet when he smashes.  Maybe make him jump forward more?  I know in my edit, he kind of floats up (but hey, he can fly..), but keep in mind that the player will need a bit of time to register that he is using that attack, so they can get out of the way!