This one is pretty straightforward, but can get a little bit harder around 5 or so.
You must kill 10 zombies total within a single game, and all of those zombies need to still be within any of the 4 stores in the main junction area.
The four stores are
Chang's Laundry just to your right outside the first gate.
You cannot board the windows.Wholesale Sugar just to your left outside the first gate.
You can board the windows.Zac's Hats further to your right near.
You can board the windows.The Ice Cream Store much further to your right, near Stamin-up.
You can board the windows.My method was to spend the first couple of rounds point-whoring and attempting to kill any zombies tearing down the boards of wood within the 2 further down stores, as you can see into both of them easily. As I got further up in rounds I eventually bought the Kuda and Juggernog so I could live a bit longer.
This basically has you in a confined area at the beginning of rounds shooting into windows so be prepared for the worst to happen.
I found that at round five at least 6 zombies immediately came out the one furthest store which is where I ended up getting the trophy.
Just be diligent, watch your back and kill quickly.
Here is a video by PowerPyx showing you how to get the trophy