RAGE

I did this immediately after achieving the 'Wall Hack' achievement, it's nice and easy;

When you finish the first Scorchers mission (the Hagar Caves), you are given the Nailgun, which will get you both the Railgun and Rebar ammo achievements.
After you've left the caves, head over to shop in the Hagar Settlement (down the central ramp towards the exit of the town), here you can buy Rebar and Railgun ammunition, get both.

Now, when you've entered the next area (the Refinery), there will be a group of 3 enemies around a fire, simply load up your Rebar shot and shoot one of them, this should pin the enemy to either the wall or the floor, popping the achievement.

Thanks to McoreBE for the video guide;

When you enter the Refinery, you should encounter a couple of enemies. They are easily noticed because of a big enemy with a big gun and tubes on his back.

After you've dealt with them, run upstairs and you should see another guy on the other side of the map (still inside the same building). Kill him, then run up the ladder which'll lead you to his location. Look around you and you should see yet another enemy (he'll be in some sort of 'balcony'). Shoot him with the ammunition called "Rebar", pinning him, rewarding you with 15G in one of the more playful names for an achievement!