What you dont see in that type of videos is, that every tiny mistake means you die. After 3 deads, you can go and wipe. It look's easy, because they are the top 5% (WoW) players world-wide (and they had 600+ tries to learn that fight). They are TOTAL undergeared for that kind of fight.

You can be sure, even if you think it's easy - it's not. Every of them has to play at their maximum, without any mistakes and not wasting a fraction of an GCD.

Dont forget - as much undergeared as you can be, so everyone has to be at the maximum to dish out the needed HPS/DPS to win that fight.