Sounds hard but really isn't if you have semi decent players.
This achievement is possible by yourself however this achievement is much easier with 4 players. Microphones are a must for this! Coordination is crucial in order for this method to work as well.
You will encounter your first survivor on round 6. At this point in the game you should have at least your exo suit, some semi decent weapons and possibly jug/exo health (enough to get by).
I recommend the team spend the entire game time in the value voltage room (quick revive/exo revive side of spawn and the room with exo solider). The team should run around and/or camp if you wish in this room, there is a MPL located on the roof of the gas station and i highly recommend you to keep buying ammo so you can earn more money for better weapons and essentially survivor there for longer. When it comes around to the 'area of poison gas' rounds i'd recommend going next door to the atlas command room (the opposite side of the value voltage room) just for the round (make sure you end the round in the value voltage room!! This is very important for survivor spawns).
When the survivor spawns
do not approach it until all of the zombies have spawned (i should also clarify that all players will need to be located in the same area for this to work efficiently), do not shoot any of the zombies for the sake of the achievement, if you are cornered at any point during this process do not shoot and take one for the team. Make sure at the start of the round all players are in the same area to avoid random zombie spawns.
Once all zombies have spawned get 1 person (doesn't matter who) to go to the survivors spawn location (make sure no zombies are following you).
The survivor should crawl out of the crevice he was hiding in and begin to walk towards his pick up location. Follow him all the way towards his objective killing anything that comes near him even though nothing should be near him because all the zombies would be with the other 3 players.
The survivor will be picked up by the helicopter and you'll receive a free weapon upgrade the following round.
Rinse and repeat for the other 3 survivors (just a heads up the round before every survivor round is an area infection round (areas will be covered (usually burger town) in poison green smoke)). A survivor spawns once every 5-6 rounds so..6-12-17-22-27-32 etc so you'll wanna do this right the first four times in order to avoid failure on the higher rounds.
If you're having any problems with this achievement post a comment or PM me and i'd be more than happy to help. Also if you dislike the solution please tell me why.
Happy Hunting!!