This can be earned on any level that has intact cars, and enemies that use them for cover. Cars blow up very easily in this game, so it shouldn't even take a full clip. Just wait until 2 or more enemies are directly next to a car, and fire at the engine/ sides. There will be many opportunities to pull this off.
As early as "The Hog", you can use an RPG-7 as well. Fire it near the bottom of the side of the car when enemies are hiding behind it. If you fire too far off to the sides, the RPG blast might claim the kill instead of the car explosion.
The mission "Death From Above" also offers many opportunities for this. When there are many enemies near a vehicle, spray it with the 25mm turret to blow them up. Don't use the other guns for this achievement or the gun blast may kill them instead.
Feel free to ask any questions, and add me on TT/ TA/ PSN/ XBL as Danokitty. Happy hunting!