Kill 75 enemies with the rifle (This includes wolves). I popped this on a wolf kill. I got this with no issue, and I was just rotating through weapons, using what was best for the scenario. Once I got other weapon achievements, I used the rifle a little more, but you are forced to use it in several parts of the game, so it should be easy.
It has been confirmed that the Rifle Grenade Launcher attachment, as well as Rifle Executions count toward this achievement as well - thanks to Sampa1 for confirming.
Work on getting the weapon upgrades and it will reduce recoil and add more ammo to each clip, making the weapon more useable.