You must complete all six races with each car available, which means you'll be playing each race about 6 times each. You can attempt to do this legit, or you can try the following method:
1) As soon as the race starts, get out of your car.
2) Go around and find whatever you can to make a blockade.
3) Stand about 100 yards ahead of the blockade, on the left side of the race. The cars should pass you on their left before they get to the blockade.
4) When the first car passes you, pull to lock onto the car and hit left to lock onto the gas tank. A single shot will send the car into flames.
5) You may be able to get the second or third ones before they crash into the blockade. At any rate, you should get about 5-6 cars the first pass.
6) Don't chase after any cars that slip through, just let them go.
7) Rebuild the blockade
8) Repeat steps 3-5 until you're the only car left in the race. Then, just get into your car and complete the race at your leisure.