As pidaa83 stated before:
Do not use any Aether/Stasis Dmg!

I really struggled with trying to not hit the boss while Transforming since I used either AOE DoTs or delayed dmg
I also didn't want another player to speedkill him for me
So I followed pidaa83's advice, got rid of all my aether dmg sources et voilá the boss did not enter the bugged state and I was finally able to beat them all by myself :D
I cannot say with a 100% certainty that this method will work since I only tried it once but I'd say it'S definetly worth a shot!