I was playing and the game died at destroying the first seal; Twilight asked for her and the princesses to be left alone for a few minutes and then the game went to a black screen and stopped responding to any key presses.
(Also, it seems like the costs of some of Celestia's skills are wrong, unless I'm missing something. Mass Light costs only 1 mana despite hitting all enemies and dealing a damage type that all bosses seem to be weak to, and Raise costs 240 mana despite it being impossible for Celestia to have more than 100 mana.)