Cool concept! Was not expecting to play it for 30 minutes but I did, made it to Defiance 26 with ~39000 points, though I got bored around Defiance 20 and stopped trying. Not to say the game is boring though, far from. Just think it gets a bit same-y after a while. More enemy variety and some more upgrades and abilities would help, though!
I loved the action of the hammer swing, so juicy with the sound and the screen shake and the cool little pattern, every swing was a treat.
A couple of mechanical issue I had: I noticed the special move (forgot what it's called) you have to wait like a full second after your last swing to use it, which didn't feel great, led me to using it less that I think was intended. I also feel like having multiple enemies become red in the same tick feels unfair, especially the square guys. They could generate 3 or 4 red tiles at once, often on opposite sides of the screen, so you basically couldn't avoid damage. Maybe that was intentional but I didn't love how that felt. But overall, still loved the game.
I think there is some real potential here for a whack-a-mole roguelike. Not sure what that would entail, but I'm here for it, great job!