I'll be honest, I did not have fun playing this game.
Here's the deal: When making a metroidvania, especially one that deals with guns and platforming (all of them) here are a few basic guidelines:
1. Always assume people want to sprint. Turn it on, always.
2. Nobody likes a slow gun.
3. All enemies must be animated (this is a general thing for all games, actually.)
For you, it was:
1. The sprint button was annoying, and it made platforming a chore. Especially because the jump and sprint buttons were next to each other. At least make holding down the shoot button make you run faster.
2. Your gun was painfully slow. Even from the get-go your first gun needs to at least have some speed to it. Feel free to increase enemy health as long as it just feels like my gun is more responsive.
3. Your bats looked like what I used to put in my crappy GameMaker games.
Otherwise, this game has potential.
If you ever decide to make another game of this genre, keep those ideas in mind. I think CaveStory is required playing if you're gonna make a Metroidvania, so go play that, it's free and will teach you a lot about how to make things feel "right."