I've completed the game. Turns out I dropped it right before the boss battle. So, boss fight is ok. I pretty much boils down to just stand in the right hole until demon is ready to be punched by a crate. It was really hard to tell when I can hit the demon. A lot of my attempts were wasted because I peaked from the hole, thinking it's time to kick only to die because of ray or miss with box, because it was just moving.
But! I would like to see this game finished. You should invest time in learning trenchbroom and it's integration with Godot.