First, purchase the "Saboteur" perk for seven perk points. Then, stand on any road near a fast travel point, in a hostile territory, and wait for an enemy car. For example, try the "Red's Farm Supply" fast travel point in the middle of John's region. Enemy cars always spawn there within a few seconds, but any other fast travel point works too. When an enemy car appears, wait for it to stop and shoot the driver. Next, stand by the driver's door and sabotage the hood of the car (hold X). The remaining enemy cannot hit you when you stand on the side of the hood rather than the front. It takes a few seconds to sabotage it. Once done, crouch around the car. Make sure the enemy stays nearby (he usually takes cover behind the car if you crouch). It takes approximately 10 seconds for the car to explode. It has to kill the enemy (he has to be very close to the car). Once the enemy is dead, open the map and travel to the fast travel point again. Stand on the road again and wait for the next car. One usually spawns within 10-15 seconds after reloading the fast travel point. Repeat this until you get the "Explosive Surprise" achievement.