I'm not sure to need there a teammate or not. I did with teammate.
You need set up x4 zombies.
Play at 'GATEWAY TO HELL' map and go a little bit forward until you reach this section.
When the zombies are coming you try hide the wall and box between.
Good luck.

P.S. Maybe you can get with this guide the other achievement:
Come and get it! It's a running buffet!
Get at least 20 for 1 explosive kill in 4 player co-op