>>717271537
Don't be too hard on yourself, the Barrow is a difficult location compared to how early it can be accessed and he's the crown jewel there. So what do we have here (from top to bottom)?
>extreme STR but average/slightly above average attributes otherwise
>thus Reflex (+5) Saving Throw is the lowest, Will might also worth a shot
>only 13 Touch AC
>constant 3 Damage Reduction (DR) but 5 against Bludgeoning attacks
>Immunities are what they are, keep them in mind
I don' want to go deep into this but Inspecting him gives you a good idea of his weaknesses and how they could be exploited. The DR only applies to non-magical attacks from weapons (hence why you couldn't injure him) but enhanced weapons (+1/+2/etc.) could overcome it. The solution? Magic!
He also has high BAB so he'll land his attacks (especially with Power Attack and Reckless Stance; Cleave for multiple targets), and thanks to his Frosty (+1d6 Cold) Greatsword +1 he hits like a truck. A pure (sky-high AC) tank with extras (Mirror Image etc.) is your best shot with a secondary on the side for emergency.
Grease from a proficient spellcaster (high casting Ability, possibly Spell Focus or even Greater SF on Conjuration) indeed goes a long way here as it solves multiple issues, but a Scorching Ray (targets Touch AC, so it's easy to land and bypasses his DR) would've also been devastating. Specialization often pays dividends tenfold.