Desperados III

This was the first achievement I was able to get in the game, albeit, through reloading my manual save.

In the 2nd mission (or 1st after the Prologue) "Running Late on Payday," you travel your way across a train that has been stopped by bandits. The first companion you'll encounter is Doc McCoy, but you must save him from two bandits first. After saving him he'll join your posse, and you can start learning a couple of his abilities. You first need to use Doc McCoy's medicine bag ability to lure one or two of the bandits in that area. After the two bandits are eliminated you are next introduced to Doc McCoy's Colt Buntline (Sniper ability).

Make a Quick Save.

In the very next area you will see a Bandit perched up high on a cliff overhang. If you haven't already, use LB to select the Colt Buntline, press down on the Left Trigger to bring up the Sniper Reticle, use the Right Thumbstick to move the reticle over the Bandit and press X to fire. Stop, Reload and Repeat.

*Note: My tracker would only show progress after every 10 kills or 20%.

As with most of the cumulative achievements you can simply reload a save you had prior to sniping an enemy with McCoy. Just for reference I completed the story once through on hard and then had to go back and get approximately 25 kills with McCoy's sniper through this method.