Keep heals on that tank because they will be taking more damage than the other tanks. The poor extra tank also takes stacks of Doom Strike which does damage when the stacks fall off, so the tank needs to somehow trigger it at regular intervals to avoid taking a whopping big damage. These Marks of Doom will keep coming the for the whole fight, so just keep away from everyone whilst you wait to get hit by something and make the Mark of Doom go away. If you do take damage, you get a Mark Eruption that deals big damage to all players within 20 yards of you (the purple circle). The crappy thing about the Doom Lord is that he makes these Marks of Doom on a few people and after 15 seconds if those targets do not take damage or have these Marks dispelled, they will insta-die.
When the curse wears off or is dispelled, a Doom Lord appears and that is what the third tank needs to tank. When you start the Mannoroth encounter, when the mob at the red Fel Spire is up a Curse of the Legion is put on someone. For this achievement you really need 3 tanks.
