Another banger of a game that I'll be sure to come back and play again, I'll be adding this to my Best of the BTP Game Jam Collection!
First of all I wanna say that this is a really polished game and I really like the charm of the game art. This reminds me of the first game I completed making, or at least what I set out to make, but this is a lot better and you've inspired me to make a similar game in the future.
I've seen a lot of games employ the less health equals more ammo/damage and while I would usually mark you down in innovation for that, I think the way you use it is pretty well designed. The fact that your ammo counter carries over to the next level unless its before a boss battle makes it so in each level, the player has to carefully consider how much ammo they use in each level since they won't know if they'll need a certain amount for the next level. This especially got good after the first boss battle.
As for the boss battles, I think they are very well designed. The slime boss battle was very well designed, with the enemies on the side to ensure that you don't run out of ammo was a nice touch. I did like the final boss battle better though and what a twist! I didn't expect to be versing myself and I'd say the way it was designed was genius. The fact that it adheres to the same mechanics as the player makes it good as you understand its limitations/weaknesses and how to exploit them. On my second playthrough, I just dodged his bullets, stole every health pack and just waited for his health to go down.
I think most of the enemies were designed well too. The two gun enemies as well as the skeletons were very nice as the stationary type enemies however, I think the mobile enemies need some work. Because they target the player as soon as the level is loaded, it doesn't give much time to react, especially if the entrance to the level is narrow. I would perhaps add some sort of range detection, where they only start going towards the player once they are within a certain range. I think you could've done some other moving type enemies where they move side to side or something and shoot just to add some more variety to the enemies.
The player controlled very well and I don't really have much complaints with it however, and I might be imagining things, the distance of the dash seemed to vary from time to time.
Overall, this was a great game and one of my favourites of the jam. I would very much like to see this expanded, with more enemy types, maybe puzzles and I'd love to see what other bosses you could come up with. I'll be sure to play this again once I finish rating some other games. Amazing work!