This achievement isn't terribly difficult if you're fast. Simply mount a caragor and sprint around the map looking for groups of slaves. The slaves will appear as small black arrows with 1-3 uruks (large red arrows) nearby. Avoid strongholds since you won't find large groups of slaves and they can be difficult to navigate. Caves, crop fields, and rocky outcrops are good places to look.
Once you are in range, draw your bow to enter focus mode and line up headshots to take down the uruk guard(s). You don't have to hang around once the uruks are dead, just shoot them and sprint off to look for the next group of slaves. If you run low on Elf-shot, drain your mount to get more.
Keep sprinting around the map until the achievement unlocks. I completed this achievement on the Udun map, which is a little easier to navigate on a caragor.