I got to Satan on my first try, and midway through his first form, the music broke and started playing really slow and loud, like some kind of horrible grinding synth. Then, the framerate dropped to probably around 5 FPS. It was literally unplayable, as it took a few minutes just to move from one side of the screen to the other. After the headache of fighting through such horrible play conditions, I got to the final form and would have killed him, but when he had about 1/4 of his health left, the game just instantly quit out.
I can't imagine I'm the only one who had this issue, and I really hope it gets fixed, because otherwise I won't be playing anymore of this game, knowing that that will likely occur again.