Oh wow. I suppose I really made it way too hard this time. Before I adjusted the difficulty the boss was actually way too easy that one could just stand in the corner and fire the Assault Rifle.
Your boss' AI is actually super good, but I think that if you give players like 1-2 seconds of after-hit invincibility while moving around, they can maneuver themselves around without being stun locked.
P.S. I got pwned even hard when I used the shotgun. XDD