I did it in the cheesiest way I have ever done a boss fight.
First 3 or 4 tries, died very quick, got him to phase 2, but got oneshot.
Then I accidentally noticed that I could summon the companions there while riding a horse, I had been dismounting. As I was running around, I noticed that the companions respawn at various places across the zone.
I just ran around on the horse like a madman spamclicking as I ran over the golden summon signs. Eventually he died while I was halfway across the zone from him. Then they praised me as a fine warrior. I think I hit him once.