I thought it was impossible too - but you can do it! Make sure to taunt with Gully and then cast Heroism. Sunder once with Garri so that you can stack Rupture. This will be your main source of damage. As long as the boss is taunted he's managable - he shouldn't do the AoE poison. Defend on Gully until Taunt is gone, recast Taunt and Heroism, then Defend again. Clean poison with Bursts from Bretto, cleaning flasks and/or Warrior's Resolve.
I ate a piece of blue food that gave me +42 health or something per tick before the battle (bought from the bar in the same location as you find your fourth companion) and obviously that also helped with mitigating the damage.
You can do it! edit: Exentryk posted a great video:
https://www.youtube.com/watch?v=7qWLlk2Ljj4&feature=youtu.be
You guys saying Knolan isn't worth leveling then? I brought him along for the third dungeon and got him up to 12 as the rest of my team, but I might ditch him. Sure he has some spells, mana restore and an aoe heal but meh... I miss the sunder and awesome perks from Bretto.