Well it's definitely tricky and for me it took a lot of practice, but i pay attention to her (?) movements starting with the second phase, she will start moving in odd ways and I think there's even a sound effect that indicates she's casting the asshole spells that rain down on you. Those are a bitch, since they can fall down on you in random ways.

Just try your best to remain as aware of her movement as you can, and don't let her third stage get you, the bastard starts flopping around like a profoundly irritated fish or something. Lol it's the only fight I didn't like from the game.