To get this trophy "The Free Folk", you must complete a mission that's called "Outcast Rescue Mission."
First off, you need to find a uruk nearby with slaves working. If you see one of those slaves that has a fist on top of their head, that means a new mission can be unlocked. It doesn't matter how you kill the Uruk (or two or three, depending on how many are surrounding the poor slaves), but once that's done, that specific mission should be unlocked.
Go to that mission's starting point and load up the mission. It doesn't matter if you complete the bonus objective or not, as long as you free the main slaves, it will still get the job done.
I've attached a video guide, courtesy of PStrophies. This should help you explain, through visual detail, what you need to do. Have fun and good luck!