There's an in-game warning not to go into the throne room because you're only going to find death there...
Yes, he's a beast of a boss. But he's totally optional. In addition, as players have noted above, the right equipment mix makes him go down pretty easily, well a little easier (High def, high fire res, death damage). He's not likely a boss you'll beat the first time you come across him (Level 13 to Level 15) but he's something you should be able to handle by the time you start to explore the the Arc (Level 18 to Level 22, with some arc stuff like equipment and potions).
The King of Korne is comparable to the Void Lord, but might be a little more difficult to separating from his attendants than the Void Lord.
Using good tactics, such as luring King of Korne, or his attendants out of the throne room, makes the fight a lot easier. He's not the first boss that has well placed guards so nothing new there. Also, trying to rotate between him attacking the main character, a companion and a summon will help spread the pain more evening (and reset his duel counter).