Ok, so I beat the game on hard mode, and I have some suggestions/ideas.
1) Let people navigate the menu using arrows instead of just WASD. I spent a few seconds wondering if the game was glitchy.
2) Add some kind of animation between boss forms, the changes seem really abrupt
3) Make the bosses follow you around, instead of staying in the middle of the room. Most of the attack patterns are easy to avoid from faraway, and you can just whittle down the boss's health. Having them follow you would make the game a bit more difficult.
4) Introduce new enemies over time, instead of just throwing the player into level 1 with multiple types. It would let the player learn to differentiate them a lot easier.
5) Vary the enemies. With the current artstyle and chaotic nature of the game, I can't tell the difference between one enemy/boss and another.
6) Make the player and enemy hitboxes match. The player only takes damage if hit near the feet, but hitting an enemy's head damages them. It makes the act of dodging very jarring.
7) Add a storyline to explain why the player's hitbox is only at the feet. Maybe they have a demonic shadow that transported them to the underworld, so that's what the demons target. I don't know.
8) Add some kind of upgrade/ability tree, to make each runthrough feel a bit more unique.
9) Make the entire game 30% faster. It's not hell-ish enough. Everything is moving slowly enough that I can plan my moves in advance; make it more hectic.
This game is clearly a beta version, but it shows a lot of promise. Hope this helps.