I don't think I have much to add that hasn't already been said; the visuals and sounds are fantastic, the worldbuilding is great despite (or because of?) how minimalistic it is, and the boss is too hard.
But I do want to talk about the boss, as someone who spent way too long on it, got very close twice, and still couldn't beat it. I don't think it's technically too hard. The attacks are all very simple to understand, and when the player is prepared for them they're easy to dodge. The main issue is that it's hard to prepare for them since the boss doesn't really communicate when and where it's going to attack. If you're going to keep working on this (which would be great), then try adding some kind of warning to the boulders and indicator that the boss will start charging soon before you tweak its stats or timing.
Also I think a down swing would have been a great addition since it would give skilled players an opportunity to attack the boss while it's in its charging state.