I've done Veteran of the Wrathgate some four times and every time it seems that the Lich King isn't so tough when it comes to the plague developed by some renegade apothecaries from the Forsaken. Looking at how it all ends at the Wrathgate, I think Alexstrasza is quite a match for Deathwing later on when it comes to if he is tougher than the Lich King or not.