The melting animation after shooting a demon is awesome - it caught me totally by surprise. Even after I knew it was coming, it wowed me almost every time.
The gravity implementation feels a little wonky. The button-hold to jump higher works great, but falling down from a jump is way too fast. It doesn't impact my ability to play the game, but it did feel a little strange. The shooting is so rhythmic, it feels like I should be able to time my jumps perfectly to hit midair targets, but it doesn't seem like a plausible strategy for a mediocre platform player like myself.
The game does feel very challenging, in a good way!
The startup sound at the beginning of the stage adds good tension, especially when you can't move yet, but there is a visible demon right behind you.
I'm not sure if the "penalized" sound bite fits the psychedelic aesthetic, but it made me smile every time.