This achievement can easily be done solo.
Start any map you want on beginner difficulty and play until you get enough money for the "Zed eradication device" and a secondary(powerful) weapon to kill zeds after you have slowed them down.
To slow down with the Z.E.D. gun hold down middle mouse button(secondary fire) and hold until zeds are covered in white mesh-like effect. Quickly switch to your secondary gun and kill them.
Alternatively you could download an achievement map, buy the gun instantly and complete it much quicker while spawning trash zeds. Something like this:
steamcommunity.com/sharedfiles/filedetails/?id=117243378