I've beaten the Arishok with a mage Hawke at least four times, by this point. (Adder's had three runthroughs.) At least one of those was on Normal. And I didn't even have to runscript healplayer my ass out of the fire on the Casual runs.

I go Primalist, Elementalist, and Force Mage. Freeze him with Petrify and WG, then throw down Gravitic Ring and every AoE you have. Tempest, Chain Lightning and Stonefist are my gods.