Huge pain in the ass.
You get this achievement for taking out 2 reinforcement car drivers as they are coming in, before they park. This is after the chase scene where you obtain the "Out of the Frying Pan" achievement.
First step to help would be to download the Chicago Piano DLC, still free as of this posting.http://marketplace.xbox.com/en-US/Product/The-Chicago-Piano/...
Now I would go to the games control options and turn the Auto Aim to high as this will help negate the recoil while you are aiming at a moving vehicle. Its important that you make the chase in a police vehicle as the Chicago Piano will be in the trunk.
Now all that is set up we start hiding behind the car with 2 guys shooting us and a car pulling in from the left. Now you can't hit this car but there will be 3 others we can make the attempt with, remember we only need 2.
Now get out of cover go to the trunk of the car and press A select the Chicago Piano (it looks like a tommy gun). Stay behind the car you came in on and kill 3 of the guys, now turn left (direction you came from at the end of the chase) and a car will be coming wait until you can see the driver and then zoom in fire away if you are successful you will hear a horn. Now turn back around and run towards the parked truck in the back of the compound another car will come from the left use the same steps from the last car, if you were successful both times the achievement will pop now if not the last car will come from the right so spin around and try again, he comes really fast so make sure you do it as soon as you can.
If all three cars are stopped and you still have no achievement let the left over people kill you and try again. It shouldn't take more than a couple tries using this method. If you or your partner already killed everyone and have to go into the building quit to the menu and reload it will take you back to the apartment that is doused with gasoline.
EDIT: Found this video skip to 5:30 for his method of obtaining this achievement.
As with all my solutions please post a comment if you feel there is something wrong before voting negative.