First, what you have to do is to hijack a car (from the garage), or a truck from the maintenance tunnel. What ever you do, DO NOT get a motercycle. It just hurts you. Run the car (or the truck) into the jeep multiple times.Most times you just ram into it once and they die. It easier to use the truck. It is bigger and more powerful. Now, this should kill the passenger and the driver easily. Oh and by the way, this manuver does not hurt you. The guy on the gun is a bit more difficult. You can use the truck (if it didnt get destroyed yet, or you can take out a pistol or katana. Five shots for the pistol and five swipes for the katana. When he dies(if the jeep isnt destroyed yet), you can hijack and you will get an achievment. If not, you can take the turret. It is very powerful.