Call of Juarez: The Cartel

Load Chapter 6 where you are taking Jess to a new safehouse. This is one of the rare opportunites you can shoot from a car.

Ensure you choose a sniper rifle, accuracy is the key. If you spray the car with bullets it will explode and the people who are blown up inside don't count toward the achievement.

Also, choosing Kim helps as she has the best ability from long range.

May take 2-5 attempts based on skill but you'll get it eventually.

I managed to get in a car at the end of level 7 and just hang out the passenger window and shoot about 10 guys. (remember you have to do the game three times so you don't need to keep replaying level over and over again for no reason).
If you use this solution and the other level 6 solution (PJDONNELLY1) then you should get it prob on your second character playthrough.

The car was just at the entry of a car park that you should go through on foot. I jumped in the drivers seat and pressed right to jump across to the passenger seat, then once more to pop your body out the window.

You know when you are in the car I mean because if you drive the car across the parking lot then you will crash into lots of stuff. First thing you drive into if I remember is a big trash bin.

I got this on level 2, but you can get this on any level with one or more driving sequences with enemies. As long as you don't start in the car, but have to press X to drive the car, your AI partners will seat themselves in the backseat leaving the passenger seat free. Whenever you encounter enemies on foot or in a car, simply hit X to apply handbrakes and come to a full stop, then press right on the analog stick to move over the the passenger seat. You can now simply shoot out of the windshield or press right once again to hang out of the window.

Once you have shot the enemies and are ready to drive further, press left on your analog stick until you are back in the driver's seat.

You should probably do this on easy as you will be a sitting duck for the most part and will take a lot of damage.

There is a way to get loads and loads of kills really simply.

In Chapter 5, once you have to collect the money and take it to the car, you can sit there and kill loads. Here is how.

1. Play on easy with any character.
2. Build your Concentration Mode up.
3. Activate Concentration Mode and reload checkpoint (now you are pretty much invincible).
4. Get to the point in the video below - your car has crashed.
5. Kill the first group of enemies until you can collect the money.
6. Take the money to the getaway car but kill no one else.
7. Get in the car and press to change to the passenger side.
8. Lean out of the window and fire (many will spawn in front of you)
9. Rinse and repeat as necessary. All progress is saved, even if you die.

I got 15 or so in one go.