Be extremely careful when battling him with heroes and/or henchmen. Because of their nature to clump together, Korshek's Liquid Flame and Searing Flames spells can wipe your entire party within seconds. Ordering them to spread out before confronting Korshek will help immensely.
Since all of his skills are from the same attribute line, one hero with Power Block shuts him down completely. With Domination level 15, it lasts for 12 seconds, which is more than enough time for the rest to kill him.
Shelter and Extinguish are highly recommended here. Shelter will prevent a catastrophic spike to your group from Liquid Flame, and Extinguish will keep the direct damage from Searing Flames from processing. Another good idea is to daze Korshek and keep a couple melee fighters attacking him and most of his spells will never get off.