Some good ideas and cute art.
I like how the bosses get faster and their attack patterns more complex as they get lower in health. Healing and attacks using the same ammo is interesting but could respawn slightly faster.
In terms of more detailed feedback:
The player's hit box is too big as you often get hit by attacks that shouldn't even touch you, plus some attacks are basically unavoidable such as the tiger running at you then hitting you. The melee attack is also pointless as in most cases if you are that close the boss is hitting either through an attack or simply via contact damage.
Overall with a little more tweaking and fine tuning could be a really good game.