Cool minimalist platformer! I like the small details such as the bushes and the squish animations.
The boss was indeed difficult but I was very glad that there was a checkpoint there. Speaking of which, it'd be nice to have more checkpoints across the level, I think, since everything is 1-hit-KO. I eventually beat the boss after about 6 or 7 tries.
Another small feedback would add coyote jump (or, if it's already there, make it a bit more lenient), because sometimes I would try to jump off a platform and just fall down into a hole. Oh, and another minimal one: I'm a dirty WASD player (my mini keyboard doesn't have arrow keys) so I had to switch to another keyboard to be able to play. Not a hassle at all, but something to consider.
The sprites and animation for the boss were real nice, the visuals overall are great. The controls felt good (other than the thing about coyote jump). Good job!