Unlock the Real Mega Buster, then shoot all three convicts once in this order: gunner, driver, passenger. Get in the car and you will get the achievement. Alternately, get the submachine gun in the Al Fresca Plaza Fountain and wait until the convicts crash into a tree. Shoot the gunner with the submachine gun until he dies. Take the gun he was using from the back of the car and use it to kill the driver and passenger. You may want to do all this during daytime and/or after you escorted Sophie or any survivors with you.