One brave Death Knight has beaten Supreme Lord Kazzak by himself which was normally designed for a 40 man raid. However, there is a small technicality that the more people are attacking him the more HP he will get. Doing it solo, though still leaves him at around 40 million HP which is quite considerable and rather difficult to defeat.
The Death Knight Raegwyn is no stranger to fighting particularly strong NPCs he has almost made it a personal goal to fight the strongest NPCs solo or so it seems from his channel. However, defeating Supreme Lord Kazzak is truly a feat that should be recognized for the simple fact that he is a world boss and has a pretty amazing loot table that people would love to get their hands on. Not to mention the fact that he was lucky enough for players to not join to inflict damage on Kazzak as that would further interfere with his attempt.
Speaking of attempts, it apparently took Raegwyn 20 attempts in order to take Supreme Lord Kazzak down.
This Boss is very challenging. The Challenging Part is that people tend to come and attack it in the last few % which leads to me having to start all over again. (had to make more than 20 trys just because of that reason God Dammit)
Jokes aside, I tried this Boss on PTR already and everything went so damn smooth that i thought its killable right away. But i didn´t think it would be possible to solo down 40 million health without random people joining until like weeks after release. Patience proofed me wrong.
As you can see it´s a simple Tank and Spank Fight which doesnt require much aside from using CDs whenever you dont have the Mark and some Death Strike Timing when you have it.
Mark Of Doom + Death Knight Healing have just perfect synergy and make you basically unkillable.