To unlock the Efficiency trophy in RAGE 2, you will need to kill five enemies without reloading.
You can do this very early in the game when you pick up an Assault Rifle as part of the story. Once you have the Assault Rifle, continue to the next area where you will face some easy enemies that only take a few shots to kill. Aim for headshots and you will easily be able to kill five enemies without having to reload.