Ok this ones actually a lot easier than you might think. Load up a single player game on chapter 2 O(Cathedral of resurrection) with the difficulty set to easy.
Once you get to the first safe house , turn left and walk down the street and you'll get to an opening where skeletons spawn out of a wall, dispatch them and any other zombies that are in the area.
Now to the left of the wall where the skeletons spawn is a closed gate that has zombies pressed against it, simply headshot them all, and it you're a little short there are loads wandering at the back.
NB. If you miss simply reload the check point as it's straight after the safezone.https://onedrive.live.com/?cid=DD524A12547220C6&id=DD524...