Eat Lead: The Return of Matt Hazard

You should do this achievement on Minimal Hazard difficulty, as the shotgun in this is INCREDIBLY weak, and takes about 3 or 4 shots (even with a headshot) to kill an enemy, and just the need to get in close to an enemy means you might die a lot more easily on harder difficulties.

You get the shotgun mid way through the second level (can't remember if it is from an enemy or it's up against a wall), Once you get it, keep it for the entire level. Also note, you might die a few times trying to get kills with this weapon. Simply run in close (aim for the chest, its a bigger target and the spread of the shotgun will hit the chest more times), make sure you are close, and fire. Kill 50 enemies with this gun, and the achievement is yours...

The easiest place I found to get this since I was playing on the hardest difficulty was during Level 4 – Backdoor Assault at the Big House. Pretty soon into the level you arrive at a tennis court where you will encounter zombies for the first time. A shotgun will spawn just to the right of the door you come in, all you need to do is just aim for the zombies heads and kill them all until there is one left and let it kill you and do it all over again until you get your achievement. Very easy, just watch out for the three Russians that are there too.

You will also get your Death Hazard achievement here too for killing 20 zombies with one hit.