Earning Passage [Defeat Rathbreaker]

Ahoi fellas o/

I've notice that when you are about to Kill Rathbreaker and move down of the inital platform to pre-kill the other wolves in circle he goes at some point up and freezes facing one direction and becomes Invincible.
He stares at one direction, doesn't move, does not attack and refills his HP which each Attack per servertick.

Heres a short video:
https://youtu.be/QvsGT0bS5nU