Okay, I feel like this solution needs some updating.
All you have to do is play through No Mercy on Easy until you reach Part 5.
Once here the host needs to initiate a vote to return to lobby. Once in the lobby, change the difficulty to Expert and change the level back to Part 5.
Then you need to gather at this point -> HERE
Ignore the part where you Molotov the side ladder, it's not necessary.
It's recommended that you don't kill the zombies that are already spawned when you initially climb to the top of the buildings from the elevator shaft; you can use them to your advantage before activating the radio. Now go to the ledge (so you are able to see the door) and wait for a zombie to bang on the door and break it down. Once this happens everyone jumps down and enters the room (one person activates the radio, then returns to the "Jesus" room). Once in the room, put your backs to the wall and wait; the Tanks will kill themselves after awhile. Once this happens (ten minutes or so, there is two tanks) you will exit this room and head east. From here you need to make it to the helipad and leap on to the helicopter.
All of the survivors get the achievement.
Credit to Crimson Carmine
for the video