Final Boss freezing during first phase

So, in phase 1 I got the boss down to 50 percent health. It starts like a black loading screen for the cutscene, but then immediately nothing happens and all I can hear is Dorian attacking in the background and I can actually swing my weapons and even see a target reticule if i move my mouse around.

Well when the black screen does go away, it's just the boss bugged out and letting us wail on him until he gets to his minimum health threshold to keep him from dying.