Neat concept! Needing to use specific weapons on some enemies was a nice touch!
I with the game felt a little snappier though: I never found myself needing the slow-down mechanic, everything felt kind of sluggish, and the hitboxes on the enemies+shots feel really unforgiving for how slow the next shot charges. It felt like I had a lot of downtime, not doing much while waiting for the next shot to charge.
Letting things charge faster, giving multiple shots at once, or otherwise making the gameplay more engaging while not actually shooting could go a long way for making something more exciting! You could maybe do something interesting by having different weapons charge at different rates as well? You've got a neat core here, just needs some tweaking and some juicing.
I'm a big fan of your character sprites, they're really quite adorable.