As you know he must be DPS'ed down in around 60-70 second max, this can be kind of hard but there is a way to give yourself an extra minute
What to do:
Start the encounter as normal, as the timer gets lower (around 10 secs before enrage) have a paladin bubble the tank (yes, sounds odd but trust me) and everyone in the raid leg it away from doomlord right back to the mountains to the south.
Keep running until you ALL find yourself out of combat (The whole time the tank stays with doomlord). Once out of combat run back in and the encounter will reset but his HP does not.
Dont know if this can be repeated multiple times but in reality 2 mins is MORE than enough to dps 1.8mil with 40 people
Enjoy the farming