Just like in RL fighting (boxing or MMA), styles make fights. I've fought Kostyan and Kornuel with a hero rager, hero storm paladin and hero squire warrior and each presented different challenges, but throughout all of them Kornuel was the tougher foe and slept = death with all my incarnations. With my paladin I could make Kostyan run within the first 3-4 rounds for the most part, but if things went bad to start off I had to get out of there pretty quick. The only option to fighting Kornuel was to bring another to permalag him and even then you probably had about a 5% chance of actually killing him, or any other imperial bard.