Very fun mechanics, but the enemies/level are not.
My only complaint is the enemy design, and that the first level feels like it should be the level 5.
The basic enemies are too well-rounded. They have no weaknesses to exploit. A lot of the fun in games for me is finding a weak-point to attack: Having to overcome groups of enemies where the enemies play into each-others strengths and weaknesses, so you have to get creative to beat them. The basic enemies have no real weakness besides being killable, but you don't place them behind a creature that makes them harder to kill, you just make more of them.. It doesn't feel like there is anything as a player to change, you just have to play better. Tt's more of a technical challenge than a game.
Other stuff:
Force the player to jump
Basic crab enemy can shoot behind itself?
The jump bullet pickup range seems a little small