This Achievement is done on Mission 3 The Heatseekers Stage 3(The Overseer)
Fight through the stage until you get to the boss fight(bounty)
You will notice lihts flashing from red to green on either side of the screen. When it goes green it means a car is on its way. Pick up the Rocket launcher and destroy 5 cars to get the Achievement. Do not waste your ammo on the Overseer and only shoot the other cars for the achievement. If you run out of rocket launcher ammo there are also other weapons which you can use and you do not have to destroy the cars in one cycle , as when they spawn a 2nd time they will have lost the health you have taken from them. You can even get this done with your starting weapon if it comes down to it.