Stage 12 featured a giant rock golem with no health bar and indeed actually invincible as I had defeated the boss and everyone else and could just keep whacking at the guy and he never went down no matter how many hits he took. He also would not attack me either so I had to exit the game and forfeit that fight. Very "fair".
The exclamation mark drawing your attention to powerups and whatever is extremely annoying with how much it moves around. Don't assume all your players have ADHD and actually like that sort of thing.
The enemy attack pattern is pretty much just the lazy follow-the-player. Pretty much the worst style of enemy "AI".
On the plus side the graphics are great and the music is fitting for a game like this. Unfortunately I'm not a fan of grind (of which there is much), follow-the-player "AI", spam (the game just spams enemies at you for the most part), flashy annoying things (that exclamation mark), or game bugs (the invincible golem, and probably others).