This is an CO-OP Only Achievement!
(Works only on Co-op Campaign and Co-op Horde Mode)
Start the first campaign with a teammate and if you're really only going for this achievement, choose "Enemy Amount for 4 players".
I chose the Webley Six Shooter Revolver as sidearm because most of the time it's a one hit kill no matter where you hit the zombie.
Go to a place and trigger all the enemies and kill all except for 10.
Let yourself go down (incapacitated) after a few hits, and just shoot as many enemies as you can.
Your teammate helps you up and continue with this method until you got it.
I think, but not tested it, you can place a c4 your friend gathers all the enemies around the c4, you let yourself getting hit by a zombie until you are incapacitated and then shoot the c4. *needs testing*