just a few tweaks would make it really good...
It held my interest until wave 4 of the final arena and then it just seemed too slow and too easy. Here's some changes I'd recommend:
1. Instead of 'X' for bash, make it the 'R' or 'F' key -- it's awkward to press 'X' in the heat of battle while using the WASD keys. Or, even better would be, click on the player to melee. Or how about this: click and drag on the player's arm to manually control the swing?
2. Reduce the number of health kits dropped or reduce the amount of health it gives... it made it a little too easy. Especially later on, there were like 4 health kits I didn't need to use.
3. Which leads to... the player is too strong, or rather the enemies are not strong enough... the challenge drops rapidly. Of course, this is on the medium difficulty, but still.. for medium, that's too easy. So a few adjustments there would increase the challenge. I think if you just made some of the enemy attacks stronger that would be enough.
4. Speed up the rate of zombies spawning for the wave attacks at the end, it was way too sluggish... and maybe make it 10 waves instead of 20.
Good job otherwise, keep it up!