Easy The White Rider Achievement

Upon arrival, make your way to the westernmost forge tower...

In the nearby river there should be a few Caragors, if not don't worry, just look around until one spawns, mount it and them move back to the tower.

Follow the route in this picture, you'll find plenty of slaves working by the main roads and local fire camps. Take note that you can kill all Uruks (you need to kill them in order to free the slaves) with your Bow without using the Caragor's bite attack, this will buy you some precious time, so make sure you are all stacked up in Elf-Shots. Remeber to consume (hold E) your Caragor to recharge the Elf-Shots.

In addition, the slaves will keep on spawning so keep going back and forth and you'll always have slaves to free.