So basically the work around for this is when in phase 2 and phase 3, you have to not hit him when he is transforming into his dark ascended form with the shield. If you hit Helmlock when he transforms from his normal self to the ascended form, he will glitch every time. So, basically wait until he transforms and wait for him to move to you (very important), once he is moving towards you for an attack, you can hit him again. I said waiting for him to move to you is important because this confirms he didn't glitch out and freeze so you can continue destroying him knowing the glitch didn't happen. This glitch is a gamebreaking glitch, but it is doable if you want to get to endgame content if you do not want to wait for them to fix it (been a week lol).
That being said, endgame is having some glitches of its own too. So I would probably hold off from the game until another patch is released, hopefully fixing a lot of the glitches that are seriously making the game unplayable.
P.S. - I can't confirm this, but almost every time in phase 2 or 3, he would transform after you shield break him. So this might be helpful to try and time not hitting him during his transformation. Hope this helps people understand how to get around this glitch some more! Great game, poorly launched sadly. ALSO REMEMBER TO FOLLOW THIS FOR BOTH PHASE 2 AND PHASE 3.