The easiest way to do this is to go into Mercenaries United and play the first level with a character that has a shotgun (because of the large field of fire). I used Rebecca Chambers myself.
Go around the map and hit a few hourglass icons to give yourself time, then run towards the middle of the map. The one story building in the middle has a stair way on the side that let's you get on the roof (or you can approach it from they alleyway behind it.) Just trail a large number of not-zombies and jump from building to the shed to the structure that looks like a railcar. Once you make your jump, use 180 spin (down on d pad and a) to turn around, aim up, and shoot when they jump. If you don't get it the first time, rinse and repeat.
Be aware if you kill too many the Executioner will show up to make this a bit more difficult.