I have a guaranteed method for this seemingly tricky achievment.
Start by having 2 other players your chatting with, as you need communication for when you all pop your ultimate.
Next up, out of the 3 chooseable characters Lahni is your best friend. As she is all melee and that is exactly how you are going to get this 9k damage needed. Make sure to have the brawler card as high level as you can, mine was only 50% Gives extra melee damage.
Edit: Thanks to Mattyl22l and mi2good4u, they pointed out you can do this on intermediate difficulty. Not only That, All players can get this in 1 run. Simply get 1 person the achieve, then all go down to restart checkpoint back into the safe room. Then go kill warden agian, and knock it out for the 2 others.
Tip:Mac's bleed card for the boltok is extremely deadly to the warden. He can solo the first few rooms if you manage your actives.
Finally, start up your match on The hive playlist under escape and get to the part where a boss will spawn about 80% the way through. It's either gonna be a swarm of little fishies,(What I call them) or the warden, dual wielding hammers. Warden is what you want. Kill him, have Lahni (person getting the achieve) pick up the hammer and run to the last room before the final room with the escape doors.
There should be 2 storage rooms on the left and right, and the way forward straight ahead. Have everyone go into the right room and wait, have 1 person run into the escape room only a few feet is needed. Just to spawn the scions and enemies. Then run back to your buddies and wait for the scions and a cluster of enemies to get very close to your doorway. Once you see the scions close enough, along with their buddies, say 3,2,1 pop everybody's ultimate and the person with the hammer unleash RB (not RT)on every enemy you see including the scions and you should net the 9k after a few swings.
Doing it this way insures close quarters which is what you need, and more safety from going down as it's a tight area so you ain't getting hammered from afar. Theres Probably going to be a very easy strategy later on, but for now ill leave this up in hopes it'll help all of you as this game releases.
NOTE:The playlist I mentioned cycles after every week or 2, so it may not be there in 8 days from time of writing this. Either way, should help some people out and get the idea that the hammer is a good option for this achieve.