Thank you for giving the game a shot and for the honest review! The boss does one final attack at 0 health, so maybe that's why it looked like you couldn't kill him? While in that state, he will shoot 20 projectiles before dying for real, but you can reduce the counter by 1 by hitting him. In retrospect, I probably could have communicated that better by setting his healthbar to 20 and reducing it by 1 for every hit or fired projectile.
The boss does gain armor during the "top" phase and the later "side" phases, meaning you will deal less damage in those phases. I wanted the axe to feel powerful to where it'd one-shot or two-shot all regular enemies as well as the early phases of the boss, but I also wanted to encourage the player to dodge a bit more in the later phases of the fight. I agree it might be a bit counterintuitive; there's a different hit sound when you hit an armored enemy, but it's probably a bit too subtle since I didn't have much time to spend on audio balancing at all. (Introducing another armored enemy somewhere earlier in the game probably also would have helped.)
The game's length (and probably some of the other issues) is partly because I made it in a week after dropping a previous project that got way out of hand. That said, I personally think it's a pretty good length for the amount of enemies and upgrades I had. I'm also used to there being a lot more entries in the gamejam, and I feel like a lot of people tend to tune out when a single game takes too long.
(That's probably not as much of an issue in this gamejam due to there being fewer entries than usual, but that's something that was hard to predict until the submission period was over!)