While this achievement is relatively self-explanatory, I'm providing the following tips and information for anyone who has yet to earn it. Happy hunting!
There are 24 total Outcast Rescue Missions in Shadow of Mordor - Game of the Year Edition. You can track your progress towards completing all of these by navigating to Start > Upgrades > Progression
Each of these missions must first be "unlocked" before becoming playable. To unlock an Outcast mission, simply look for white fist icons as you travel throughout Udun and the Sea of Nurnen. These white fists will appear above the head of certain slaves. Kill the one to two Uruks guarding/beating these particular slaves and you will get a notification along the lines of:The Uprising NOW AVAILABLE
Once you have "unlocked" an Outcast mission, you will be able to find it on your map. Unlocked Outcast missions are represented by the same white fist icon, but with a shield-like border around it (the same outline that other game missions have).
These missions are not particularly difficult to complete, but can be tedious if doing a lot back-to-back. Your objective is to free 3 slaves (but this will steadily increase to 5, 7, and finally 10) who are tied to poles. Each mission will net you ~400 XP and ~250 Mirian - can be more or less for either, but that seems to be about average. There are also bonus objectives which can be completed for additional Mirian reward (such as performing X number of stealth kills, etc.).
Try to complete Outcast Rescue Missions as you go, rather than waiting to grind them all out once you're done with everything else. I made the mistake of grabbing this achievement after finishing all others for the main campaign, rendering my earned XP and Mirian useless as I had fully upgraded everything and learned all skills already!
If you're looking to finish this one, I would recommend acquiring the Lethal Shadow Strike
) skill first - unlocked from the Wraith skill tree. This isn't absolutely necessary, but some of the later Outcast missions take place in Strongholds where being able to transport yourself quickly comes in handy to avoid large groups of Uruks and get from slave to slave without too much hassle.
It's also helpful to have your Focus and Elf-Shot maxed out to aid in using Lethal Shadow Strike (costs 2 Elf-Shot per) and for taking out, or at least reducing, big groups of enemies with charged headshots.Note:
If you're having trouble spotting the fist icons which "unlock" remaining Outcast missions, try fast traveling to different towers or from Udun to the Sea of Nurnen and vice versa. This should reveal new icons for you and allow you to finish up any final missions.
If I've missed anything or you have any suggestions for improvement, please drop a comment below and I'll add it and give credit!