One of the final missions in the game requires you to drive to the Authority Bridge and blow it up. When you drive up to the gate 3 mutants will spawn. Run them over and then drive back to Subway Town and enter. Immediately leave to the Wasteland again and drive right back and they will respawn again. Simply rinse and repeat and you are done. After blowing the hole to the area a group of them will spawn as well. in case you have a few left to get. Should not take more than 2 times killing the initial group of 3. Just make sure you stay in the car and run them all over.