| From memory Thorassus is a cleric. I'd suggest using point damage spells with Reflex saves like Ice Lance and Flame Arrow. Another good idea is to fire off an arrow if Entangelent on him. These arrows automatically entagle their target on a hit with no save. He'll be stuck in place for 4 rounds. Back off and nail him with missile weapons and the most brutal damage spells you have.
If you are still fighting him in the room where you fought the Efreet's and are having trouble with all the snakes, try Greater Command. It has 1 casting time and can disable many of them in a single casting. Then focus on killing the ones still awake. If you have a poison resistant character like a monk or Druid have them be the front line fighter against all the snakes. Otherwise have plenty of Elixir's of Health handy. Another handy trick is to summon an undead or other monster right next to Thor. This will keep him occupied for a while letting you deal with the snakes. Try Shadow Conjuration or Shades for this as they have pretty cast casting times.
You probably don't have access to Symbol: Hoplessness or Symbol: Fear yet, but if you have one on scroll of a Cleric with this domain spell, this is gold here. Otherwise, Emotion: Fear is a decent option for relaible crowd control.  | 
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