We can't figure it out, we're all level 13-14 characters: a cleric, sorcerer, wizard, druid and bard/rogue...and we can't harm Thorasskus.

Tried everything, melee, arrows, all of the nasty spells we have and *nothing* touches Thorasskus!! (first encounter). No matter what, he seems invincible to all types of attack. Even tried Outilekke's Resilient sphere (sp?) to hold him, but it fails. We tried to lower his resistance, finger of death, ice storm, suffocation, area attacks, color spray, disintegration, acid fog etc., we've run out of options- he is constantly "uninjured". Any advice?
Thanks!
Juliet