This already is a great game.
I especially love the art style of the game. The dithering of the level elements made the solid player and enemies stand out as well.
The gameplay was simple but challenging, a good combination. The self-knockback when you attack forces you to play more cautiously, and the heal charged by attacking forces you to keep attacking to survive.
As for the difficulty, I think it is in a pretty good spot. The start is pretty easy to get you used to it, and even though the hedgehogs are easy to beat, once there's a large pack of them they can still become a problem
Hitting the bats mid-air felt like a nice challenge as well.
As for the boss, he himself feels a bit too simple and easy, but the add phases make it a lot more challenging.
Still I think the boss could pose more of a threat, especially his melee attack felt like it has a very short range. Also I kind of expect a bull to charge at me at some point :P Because of that the first phase without any adds felt too easy and too long.
Finally one small UI nitpick, there is no way to quit the game on desktop ( I played the linux build). There is no option for that in the pause button and the victory screen does not respond to any button at all.