Last night, I urged 2 guildies to three man Onyxia. Unfortunately after the fight started I learned that Onyxia is immune to all fire spells. No immolate, no conflagerate, no incinerate. My rotation consisted of Curse of Agony, Corruption and Chaos Bolt whenever the CD was ready. Luckily the tank, TOC geared held aggro and finished it off.
Next run, convert to Affliction.