Zalmarath, The Colossus. Invulnerable

Zalmarath becomes invulnerable at random points. First try I got him to 90% life then he didn't take damage for like 2 minutes, I restarted the fight. Second Try I got him to 50%, same thing here, restarted the fight. On my third round now, he's at 50% and got invulnerable again. Guess I need to keep going, hoping to be able to kill him.

Update:
4th try, go him to about 50% life again, restarted.
5 try time took me about 10-15 min. Got him to about 60% life before he stopped taking damage, but now and then I could get some hits in before he got invulnerable again. Kept fighting like this, slowly taking him down. Thank god I had charge refill on my flasks.

Same exact issue. Took me forever to kill him because her randomly chooses to no longer have a hitbox at while he's attacking.

Completely unfun boss and I wasn't even a fan before.

Happened to me first run he stopped taking damage after about 10%. Second run he died after a long fight.

Same issue here.