I currently am stuck at this fight as well.
Can't break free, even with full stamina, pets keep on fighting, but will eventually die.
Everything is frozen (multiple times during the fight, actually) and even a hundred Ultimate-shots didn't kill him (brought him down to almost zero health though).
Not sure if I'm too weak for this fight, there is a simple mechanism or if this a game breaking bug.
I just did this quest and honestly, it took me a while with my Stamina-Bow Nightblade. I used draining shot on the extra enemies for healing; then I would use poison injection and Lethal Aim. I used alchemy made potions which increased my weapon crit, health, and stamina at the same time. And I used Hindering Poisons; mine just happens to immobilize him and drain health while siphoning it to me. Very helpful.
This still took me 3 times; I died a few times and had to start over. Luckily, I succeeded. But barely.
I highly reccomend an execute... Because you'll need it.
Remember to right click then left click to break free when he freezes you. It DOES work.