Unfortunately, I've not been able to find a great place to do this in free roam. I've spent hours trying to find good spots and it just never works out for me. On the plus side, this is extremely easy to get in the first yellow zone you visit in the game, Earlston.
As soon as you talk to Ned and exit the safehouse, walk west about 50 feet and you'll see a "free the prisoners" event. A single guard is standing here with his back to you, so sneak up and take him out with
. Now, run due south until you find a school bus and a couple of porta potties, and bump your character against one or the other before returning directly to where you just came from. The same enemy has respawned and you can repeat until the achievement is done.
I've also made a video of this process if you prefer to see the method in action: