Like TGV said it is simple.
First,try to aim corectly to the tires of the car/motorcycle,dont shoot in the car because you will not do nothing only you will lose life.
Second,if you cant aim good to the tires try to shoot at the person that drives the motorcicle.If you want a direct hit you should aim at his head to an instant kill.
-The Guy that shoots at you:
First you aim to him like on the driver for an instant kill at head.If you cant try shooting him when the motorcycle is aproaching to your position or when the motorcycle is on your left or right side.