This opportunity won't present itself until very late in the game. On the main quest line, you will get a quest from Portman where you need to go blow up some Authority blockades. The first one you come to there will be 3 enemies right off the bat to run over. Then as you approach to plant the dynamite more mutants will pop out. Just use your car to run them down. Finally after you plant your bomb wait in the car and as the gate explodes yet more enemies will emerge. Run Em down for this little achievement.