[SPOILER] Need help with Demons and Demigods quest

Two spells:

Spoiler

Longstrider (or equivalent movement booster) and Spike Growth/s (conjuration spells are not deterred by the magic resistance). Just lead the avatars of Iuz/Cuthbert through the affected area(s) -- they always take damage on the direct path towards you, as you run the edges of the gauntlet.

Or, alternatively -- though it takes A LOT LONGER -- Web, and with Enlarge and/or a reach weapon . I noticed this in the Arena of Heroes and Imeryds Run, once the Big Enemy is caught by the Web (which may take a while, as they only fail the saving throw on a "1"), they rarely attempt to escape. And then they are hit only with on a "20".