You can easily destroy five cars in five seconds using the Cinder Missile (Smoke Power). Fully upgrade this ability so you can carry five missiles. Consume smoke (marked with brown dots on the mini-map) to gain maximum missiles. Wait on a road until five cars are lined up in front of you. Then, shoot each car with a missile in quick succession. There are also lots of parking areas around the map. Cars spawn there randomly and sometimes you can find more than five parked vehicles in one location.