Alright, I am now positive on how to get this one. The Agitator Majini are the ones that carry around the white megaphone, and it shows a cutscene of them appearing. The Big Man Majini with a megaphone does not count. Best done on Amateur.
1) In the first area, make sure to kill everyone! And you may be sitting there a while, but keep going until you are 100% sure that no one else is in the area. Make sure to climb up the ladder that leads to a room with a safe in it, and to check the other room with a safe. And also check the bathroom area. It is rumored that you have to climb up all 3 ladders, so just go ahead and do that, just in case. And don't touch the door until the enemies are gone. Then when you go through the door, wait to climb up the Co-op ladder, and kill the guys that jump down. When that is done, Co-op jump up the ladder, a cutscene should happen and run back to the original area to find the Agitator.
2) In the area with the 2 rocket launcher turrets, when Jill has to co-op jump up a ladder and go around to let Josh in. Do not let Josh in just yet. Same as before, just kill all enemies. It may seem endless, but guys will eventually stop jumping down. When at the 2 rocket launcher turrets, if you look straight across, there are some enemies up there on that platform that you need to kill. That is also where the Agitator will spawn eventually. Use either a sniper of the turret to take him out.
3) At the end area, where you have to survive. Make sure to just keep killing for the whole time, and remember once you run out of time from the first 5 minutes, it will raise up to 2:30 again., so don't panic if you don't get the achievement before then. Be sure to kill ALL of the enemies that spawn out of the elevator, and be sure to also kill the guys that jump down (like the Reaper thing, with the arms and slashes you) For me, on Amateur, it was Red Executioner from the elevator, Reaper, Red Executioner, then the Agitator spawned with around 25 seconds left the 2nd time. Be sure to kill him right away. If you don't kill him right away, the time will expire, and the DLC will end. Flash grenades work well on the things that pop out of heads. A tip is to put proximity mines in front of the elevator, so when the enemies walk out of the elevator they hit them and get hurt. I stayed in that area in front of the elevator, that has the opening, and the window to the right when looking towards the elevator.