This is how I got this:
Go to Campaign- Dead Ahead- Chapter 2 “Locomotive lockdown”.
Set difficulty to easy and enemy setup to 4.
Now I personally found a pistol easier to do this than a sniper. I ended up using the Webley pistol but you could use a different one if you like, Anyways at the start of the level you come across a bunch of zombies that are coming out of a train. It’s pretty self explanatory at this point, just try to line them up and once you have a good shot just press the trigger.
Hope this helps!