(Edited: )

Kel'Thuzad is a convenient excuse to play the generally unplayable Onyxia.  It fits the bill of being a late game finisher (provided you have board control going into any particular turn) which doesn't get wrecked by his eventual hero power.  In multiplayer, you usually just die the turn after you play Onyxia, but Kel'Thuzad doesn't play many things that affect the board immediately.

My victory came from getting rid of his armor ASAP to stop his initial hero power before he hit 8 mana, then pulling out an Onyxia that he stole a whelp from.