I do not know if the amount of damage triggers the different stages or if they have a set number but at the end of the bossfight its bullet hell with a couple of different versions repeating. I would either lessen the amount of repetitions or if it is triggered by health, lessen the amount of health the boss has during the "stuck in the middle" stage. One of each iteration of the "bullet hell sequence" would be perfect.
Edit: I did use the grenades, but perhaps not in the intended manner.