You an earn this in mission 4. On the top floor of the museum, next to but not in the open garden-y courtyard, is a room with a single unmasked Overseer examining the corpse of a witch. This is Brother Cardoza. When you approach this area (which you can do from numerous directions) you should hear a blind sister telling him that the lead sister wants to have a meeting with him (she will then walk away). Sneak up behind him, knock him out, and use Semblance on him to take his form. Follow the quest marker out into the adjacent garden area and talk to the lead woman to unlock the achievement (and move the overall quest progress forward).
If you don't move around too much you should have enough power in Semblance left to go back out of the area before it wears off and you're revealed, although that's not necessary for the achievement.