The art & sound is amazing. I do think it needs a difficulty balance though, it can be quite tricky to survive when you're just starting. Maybe make the xp/heal drop ratio depend on player health? That said it is really satisfying when you max out the skill tree and go from playing bullet hell to being bullet hell. You absolutely need to keep that aspect of the experience.
The sheep needs a buff too, it is hilariously underpowered compared to where the player can get to.