If you are still looking for this later in the game, a sweet spot is in the abandoned distillery.
Enter and make your way through, you will drop down a hole in the floor and crawl through a pipe, this is the main mission path and is the only way you can go after entering.
After you go through the pipe and drop down back up to the wall and drop the turret and make your way forward to trigger the mutants.
There are exactly 5 that will run at you and the run and the distance between them is enough so that they will not reach the turret and destroy it.
My main problem getting this elsewhere was that I was playing Nightmare and my turrets kept being destroyed.