Grab the agency supercar from the garage, drive down the Los Muertos tunnel, and when you pop out the end, stop the car in the middle of that area. Be like 20 feet away, and shoot it with the firefly (not the hothead, nor the lobber) once, and it should pop in the air. Keep shooting, at the same time walk under it and unload your clip into it. If you reload, keep shooting it till it hits the ground. You should get this on the first try. I died a couple times because i was too close when i made the first shot. Dont use a weak armored car as it will blow up instead of poping into the air, and dont use a too heavily armored car, as it wont pop up into the air. Best use the agency supercar like i suggested.