So I have an Idea for a future boss. Why not have it polymorph you into a new school that you don't know how to play and use completely random cheats to make it as frustrating as possible? Doesn't that sound like a great idea that would just make the game so much more fun and interesting? Certainly, doing the same thing over and over again and expecting a different outcome isn't the definition of insanity!
All sarcasm aside: This boss, at its core, is flawed. You need to either get lucky, or flee until you get a good form or it glitches and you go back to your normal form. This not only makes the combat experience repetitive and frustrating, but can ruin the story too. For example, When i was leveling through Azteca I got stuck on a boss for about a month that nobody would help me with. By the time I cleared it (with the help of a max level player) i'd forgotten the story of the game completely and the quests became chores instead of progress to stopping morganthe and saving azteca.
The game has come a long way and it would be unfair of me not to point out the progress you've made. The gameplay has improved a lot and the story and characters very rich and deep. Like i'll remember how shifty Von Trap seems, but not the random mantis people in khrysalis. I'll remember how real Greta was, as opposed to a lion tribe in zafaria.
I would have had the polymorph wear off when you cast the right spell, then give you a few human rounds, then polymorph you again. So you can make progress with each window being caused by the wizard, not a number generator.
Tl;dr: This boss is an RNG check and should be changed and it ruins the flow of the story.
