I managed this fight last night, one dps died, a Ret Pally. I popped divine hymn at the end of first asphyxiate... Pally Tank died. luckily Druid brezed him post haste. And then the second one, the Ret Pally died.
I have no tricks really. I was just spamming PoH, I used Barrier trying to cut down on damage while I spammed...
Edit: got the achievement though. ^^;