I'll clear things out.
When you kill White Magister Jonathan, an invincible The Hammer's Pet spawns.
Only way out is to teleport White Magister Jonathan at longest distance, where he will fight blobs / run and burn, so this will give you some time.
Then you have to be far away (teleport Gwydian Rince), so fight will end.
The Hammer's Pet returns on place where White Magister Jonathan body is lying.
I'm getting frustrated with the game

With all these bugs there is no desire doing "for honor" mode.