This certainly was as difficult or more as everyone else said it would be. This is how I was able to accomplish this.
1. Most important, turn aim assist off. All that ever did for me was focus on someone I didn't intend to aim at. =)
2. Immediately focus on the first car that arrives and shoot the driver.
This can take some practice, but the car always stops right in front of Cole and just to the left of the red barrels and the driver's head is just under that top ring on the left barrel. Watch and study where they stop. I did not change to the sharpshooter outfit and immediately focused on the spot to aim. Sometimes you will kill the passenger, and if you do, likely the driver survives. If you succeed, the driver will have his head leaning against the steering wheel. If you fail, immediately run into the fire and try again.
As others have said, do not quit and start over, you will have to go back to the house prior to the chase again.
2. Once the driver from the initial car is shot, go to the back of your car for the automatic gun.
If you shoot at anyone, press
to reload.
3. Run down the street and look for the red car on your radar. If it doesn't show up, run back up the street a little and it will appear.
4. Once the red car is in focus, aim at the windshield and fire,
If the car swerves, it does not mean the driver is dead, it means you hit the windshield. Keep shooting.
If you succeeded at killing the driver in the car coming up the road, you will see the achievement, and congratulations. If you did not, run into the area and start over.
I had no luck with either vehicle inside and wouldn't bother with them. It was much easier staying out and getting the two drivers I mentioned.
5. If you want to, turn your aim assist back on to help with the next 2 scenes. I didn't use the sharpshooter outfit for this, but you may want to consider it for the upcoming battles.
Good luck with this. =)