By exploiting shitty AI, or using hacks (I can't think of any hacks that would prove of use, but you never know).
Say if you got Onyxia to a certain corner, and she got stuck and unable to attack, that would be a perfect example of an exploit that would nullify the legitimacy of the kill.
I guess blizzard doesn't realize how much koreans care about games... Koreans probobly killed it fair and square. Good job for those bastards... i want those items.