Simple enough, to soften her defenses you must first eliminate her 3 lieutenants. Dodge her attacks and kill minions until she flies over the arena and shoot her body. Once her body is weak enough her head will be exposed, but the process is the same. If you are having trouble defeating her, especially on legendary, I found a spot where there is a support off one of the sides of the arena where you can be safe from enemies. Just jump up when she is about to fly over and jump back down when she is out of range. Good luck.