Alternatively, I was able to beat him by dropping him out of ascended, making sure he started moving and then burning him until he was stunned and then bursting him down before the stun wore off.

I was not having any luck with the waiting method, he would glitch out on me at various HP intervals.

Had the same issue failed maybe 8 times - Staff Spell-Caster with ailments specialist, using a bunch of AoE, including Anomaly and Plagueburst and Annihilation. Tried several things based on all the feedback in this thread. Can't say with certainty what solved it for me, but I dropped Anomaly (kept Plagueburst & Annilhilation), added Solarfall, Bullwark of Dawn. Nuked through phase 1, and tried to be cognizant of the form changes in phase 2 & 3. My Annihilation beam was poison so may not have been applying Stasis (so perhaps that is the problem)

So frustrating. Clearly people have been making it beyond the last boss, so safe to assume it's a regression from the last patch?

This bug is ridiculous and gamebreaking for me! I tried at least 15 times and it bugged every try. The given workaround didn't seem to work for me... thats disappointing.

Level 42 Pistol/Shield here, 100% into toughness, all heavy armor, main stat is all resistance, siegebreaker, basically everything into defense and resistances. This fight just took me 25+ minutes on my third try. Damage bugged 4 times. I remembered reading that you can fix the bug in game by checking (or unchecking) show damage and show crit damage, then pressing accept. So mid fight with all these red danger zones targeting me constantly, I had to quickly press escape, gameplay, uncheck or check the two boxes, press accept, then the boss starts taking damage again 20 seconds later.

This fight is not fun at all to begin with. This is just icing on the cake. I never thought being skillful at navigating the gameplay options GUI was necessary for me to beat the fight.

Tip: change difficulty to "Story mode" when doing this battle makes it easier. You still need to avoid doing damage when he is changing form, but you don't risk to die yourself.

So I've confirmed a way to get him out of the glitched status. If you can somehow stun him while he is frozen, he will snap out of it once the stun is finished. HOWEVER, he can only be stunned while his shield is active for some reason, so if you can't get a stun off before the shield breaks, you will have to restart.