Forum Discussion
Had this exact thing happen when I one-shotted him down to below half health with my Tempest.
SOLUTION THAT WORKED FOR ME:
The boss is scripted, but does not have the fail-safe that other bosses in the game have, so instead of becoming invincible between "forms" he can be zerged to death before he has gone through all of his stages. Therefore if you kill him before his "final form" the game will bug out.
Lowering the DPS, and inching him down little at a time should do the trick.
For me, using the Tempest focus attack broke it for me since it took away 2/3 of his health at once, thus bypassing two of his "forms" leaving him humping the air like in the video posted above.
SPOILER:
SPOILER:
SPOILER:
SPOILER:
He has a couple of forms, and the fight takes a break at a point where you instead have to fight his dragon. After the dragon is dead the boss fight continues, and his final form starts with a cut-scene where he is standing and charging up his red orb. This is your green-light to go all out and kill him.
Hope this helped!
Bests
Neo.