I found this to be a difficult fight for a ranger. First go, I was caught completely flatfooted ... you have no time to buff, drink potions, nothing at all. I didn't even have my weapons in hand from the trial. He tore me up in no time. The only break you get before the actual battle is before you stand vigil in the temple, and any buffs or feat you use at that time will be stripped by the time you start the battle. Not only stripped, but not recharged, so you need to at least rest in the temple so you have all your abilities ready.
I had to do the actual fight at least a half dozen times before I got him. The annoying camera killed me a few times when I lost sight of where I was and had to move blindly ... until I finally caught on that I could change the camera angle while paused. In the end I wound up treating it like a turn-base battle; every action I took, I paused after, adjusted my view and planned the next moved. It was nerve-wracking and cost me a bit of sleep because I couldn't stop trying to figure out how to beat him without dragging out one of my companions

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The fight I won went like this: As he was charging, I hit him with a tanglefoot bag so I could chug some potions. The rat-bastard won a save roll and resisted it, so I settled for just a bless potion. I wanted speed too, at least, but there wasn't time, his DPS is just too high to stand there drinking potions. But it wouldn't have mattered. If it were a straight-up fight, I still would have chopped him to pieces. I had 75% health when he got to nothing, but that frenzy mode of his is the killer. A ranger even with the medium armor feat just can't stand up to his battering long enough for the frenzy to wear off. In one of my pause breaks just before he frenzied, I summoned my animal companion, and when I started getting the frenzy message, I took the advice from previous posters and started running like heck. My wolf pet wound up finishing him off, and I survived with 10% health or less. Though my character usually picks the "good" thing to do, I rejected the opportunity to spare him and cut him down after the cut scene.