The mechanic of you taking damage when enemies take damage seemed a bit odd to me at first, and in the first couple of levels, it didn't seem to have much of an impact on how the game plays, especially with the very generous number of healing items lying around.
It's not until the last level when the potential of the concept became clear: there are so many enemies and so many spike balls that the gameplay stops just being about shooting enemies and healing afterwards, and morphs into the mad dash to kill enemies, avoid getting shot, and keeping your health up. My usual strategy of chipping them away from as far as possible just couldn't work -- the system you made forced me to go running into the chaos to keep myself alive, which I thought was an interesting interaction. I would have liked to see more of the levels lean into this.
Otherwise, cute graphics and fitting somewhat-too-cheery-for-the-amount-of-murder-happening music. Good work!