This achievement can be obtained by doing the „Saving Souls“ Heroic Encounter located in the very north of Vallenhas map.
You will have to kill several groups of mobs and free 4 citizens. Those 4 citizens will count towards the achievement.
So you‘re looking at doing the heroic encounter 50 times given that you free all 4 citizens yourself and not another player. Also you have to finish the encounter for the citizens to count towards your progress.
With half decent gear you should be able to solo the encounter.
The fastest way to do this is to hop through the different Vallenhas instances by pressing „Back“ until the minimap in the bottom right is highlighted and then select „Change Instance“. Check if the Heroic Encounter is active. If not hop to the next instance.
You can track your progress by going to Journal->Achievements->Quests->Savior From the Sword Coast.