Everything is the game was smooth and juicy. The concept was unique and fun.
Lovely music and artwork.
The game was pretty chill too. I lost after 500 kills because I uncaptured the mouse cursor during a wave, and after that I had no way to make the game capture my cursor again.
One thing about the "Regen x% of HP per second", the problem with it is it works even if you don't fight and keep hiding, and the worst with it is that you also regen in menus, and the problem is that the player might wait in the upgrade select screen for 30 seconds just to wait to have full health (basically : if you give a safer but less fun way to win, the player might use that way).
Another minor problem is that the bounds are a little bit outside the screen, and long-ranged enemies can stay there while you don't see them.
Except this minor problems, that's amazing game !