This is very easily obtained in Horde Mode. In solo or with friends load up Dead End with the difficulty set to Cadet and the enemies set to 4. When the mode loads up run forward quickly and to the left to grab the Preacher then make your way back. Because you have it set to 4 player enemies you will have an alleyway full of Zombies. Use the Preacher to help clear out the Zombies that form around you and the Sniper Rifle for the ones at the entrance to the alleyway and you will have this is no time at all.
If you fail to do this at the first attempt there will be plenty more Zombies but limited Preacher ammo but feel free to restart the mode at any point.