I don't understand how you proc the reset on the first boss, or they keep fight until the end of time, or he just go back to his main stage, without any mob moving, and wipe everyone
you want enough damage on him and the controller so that they evade out (but not enough that the controller dies). you'll know they are evading when they heal. the controller will state he's regained control that's when you want to hop down and reengage. next thing is to make sure you manually break the MC before it expires otherwise adds respawn (you'll know if the door shuts)