This can be done easily by grouping five cars with a low durability rating together and using an explosive weapon. I used a car called "Purity". I found one in the parking lot near the supply point called Westport Car Park on the southeast side of the Westport region. Every time I went there, there was one parked. Once you get in one the first time, it unlocks permanently in your garage.
Near the above-mentioned supply point is a vehicle-spawn point. Once you have "Purity" unlocked, just spawn 5 of them and group them closely together. I used a grenade launcher and blew them all up very quickly. I assume frag grenades would work just as well, or perhaps any number of other weapons I haven't discovered.