A quick and easy way to get this is at checkpoint near the end of chapter 2. You will be upstairs overlooking a huge courtyard with zombies at a locked gate. There is also an ammo box to the left of the safehouse door. Take your time with headshots picking off the zombies at the gate. More will spawn and you should have about 20 after shooting all at the locked gate. If you miss a shot just reload your checkpoint. I had a chain of 35 before opening the safe house door. This also is a good place to get the 10 headshots in a row trophy. Good luck guys.