I expected a lot more
The game itself is fun, but there is a very big "BUT" here: the boss.
Each time you lose to him, your health AND you rage get reduced? WDF?!? The boss is already hard with high rage, and each time you fail to beat him, you get even LESS health and rage? So, all the work we have on the previous levels is lost because we can't beat a hard boss? That's totally nonsense.
Yes, this can be "solved" if you beat him the first time, BUT that's not something easy to do, the boss is very hard.
So you really should consider changing this. Either decrease his attack speed, or make it so that the player doesn't lose the HP and Rage he/she got during the rest of the game.