This trophy is kind of buggy, as some players have reported that it will not unlock if you use a shotgun (since the shotgun fires shells/pellets instead of individual bullets). Others have said that it unlocked for them even while using a shotgun, but just in case it doesn't work, I suggest just getting it with a rifle. There are a few methods you can use to get this trophy.

1. Just play through the game normally and you might just earn it by accident along the way.
2. Play a game of Nazi Zombies and get the PTRS 41 or .357 magnum out of the box. After you get one of these weapons, line up at least three zombies and kill them all with a single shot. This will have to be done on a fairly early round though, as the zombies will become too strong to one-shot later on.
3. There are a few good places to do it on the mission 'Vendetta', but this is one of the easiest places to get it 

 

Rough Economy Trophy Guide

Credit to MATTKBAN for the video.