Nice game, finally beat it!
It has pretty cool upgrades, I especially like the grappling hook one :)
It'd be nice if you could move just a bit when attacking, just to be able to make small adjustments to your position (like very small barely moving). And it seems like the first bullet fired always goes horizontally, even if you press up+attack. There's also a slight delay before you can jump right after shooting, it sort of makes it feel like you aren't in control of the character.
Would also be cool if you could skip the boss dialogues once you've seen them once, it gets a bit repetitive. And also maybe reduce the amount of monsters you have to kill between the checkpoint and boss. (The grappling hook shortcut to the bat boss was nice though :))
To me it feels like the bat boss switches places a bit too quickly. It just becomes RNG if you can get enough shots at him before you get spammed to death. (Also maybe the player hitbox should be a bit smaller for enemy bullets and attacks, sometimes you're hit even if the bullet doesn't look like it's hitting).
Those aren't really big issues, the game is absolutely beatable, and managing to defeat bosses feels awesome!
The graphics are super nice too, and I like the humor. Great job all around!