Metro 2033

Easiest level to do this on is Cursed. Once you find yourself at the barricade with the sandbags and the mounted DSHK, sit back behind the sandbags and kill the said number of enemies with your shotgun.

best level to do it on (if you havent gotten it by this time) is Defense or Child. Just kill the lurkers as they come out of the holes.( you will also get the lurker achievement). This strategy is also good for the headshot, pistol, knife, throwing knife, etc achievement.

Like a dummy I was blowing through this game with the automatic shotgun and it never occurred to me to use this old archaic gun. Derp.

I would suggest starting off with the prologue mission titled "Hunter". Once you pass through the market and jump on the trolley, and you get the cutscene where everyone passes out, the guy who wakes up next to you hands you a double-barrelled shotgun. You can get a few kills here and if you proceed through to the next mission you can easily get this achievement to pop.

I also kept using the right trigger and reloading (I know the prompts explain how to use a gun but they're lengthy and once they're gone they're gone) and it wasn't until a few kills in that I realized you can pull the left AND right trigger simultaneously for the full effect of the shotgun.

I know the above solutions work but if you load those missions and dont have a double-barrelled shotgun on you, you have to reset to a market and hope that specific market has that particular gun in stock.