I got this by booting up Chapter 19 -The Settlement-. Difficulty doesn't matter, but obviously Very Easy will make the journey seem effortless.
Play through until you reach the part where a henchman shoots out the support for the floor you're standing on. You'll fall into a hole, and have to shoot out another support to reveal climbing ledges.
These, in turn lead to a staircase that goes up to a room. This room leads to an open area where you'll be ambushed by 10 or so henchman. Before you head out, there is a Mag 5 hidden on the far side of the room behind a small waist-high wall. Simply pick it, up, get your kills, and reset the checkpoint as needed. Additionally, one of the marksmen in this area may also drop a Mag 5.