You get this achievement for killing three enemies with a single shotgun blast, and so far the best place I've found to do it is the level 21MEETIN. As is usually the case, you can't select levels until you've played the game through and beaten the story once. You'll just have to progress to that level naturally if you've not completed the game before.
Kill the three guys at the start and be sure to collect a pistol with at least one bullet in it. Continue forward and kill the shotgunner, then wiggle the stick a bit until the door starts to open. Hit
to toss the pistol just over the pyramid on the table to disarm the guy in the back if you can; if not, it's not impossible to get this done if you miss so don't fret. It just helps if you can avoid the guy in the back killing one of the others, and you'll notice I even missed the toss (for some reason?) in my clip.
Pick up the shotgun on the floor and wiggle the stick a bit until the two melee guys come out of the door. Try to stay as near to the door as you can and line up the shot with the rifle guy in the back at the center of your reticule with the melee guys on both sides, then pull the trigger and hope for the best. The shotgun spread is kinda random so it may take a couple of tries before the planets align correctly.
Video clip of my gameplay unlocking the achievement (though I guess the achievement overlay doesn't appear on screen in a recording?):