If they got wrecked, they didn't down "all' 13 bosses, you'd think..
Only if you're a newbie to raiding.
Especially in Naxxrammas, boss fights revolve totally around strategy. It doesn't matter how much skill, gear or balance your group brings; if you don't conform to the intended strategy (or some alternative one which serves the same roles as the intended), you'll be crushed. Since Blizzard doesn't give the slightest hint of what you're supposed to do for any battle in Naxxrammas, you're left to figure it out for yourself. There may be some clues during the trials, but it ultimately boils down to pure trial and error. You try something, attempt to observe where the group started to perish at an abnormal rate, and then try things to counter the force that caused it.
I'm pretty sure that Death and Taxes only recently started Kel'Thuzad. They've obviously conquered the strategies for all of the bosses leading up to Kel'Thuzad by finding and implementing the correct strategy. They're in the process of doing the same for Kel'Thuzad.
