'The Survivor'
'Complete the Hanging Gardens and speak with the Survivor in the Bastion'
Near the end of the Hanging Gardens level, you will find a man standing in the middle of the path. Press X to speak to him and end the level.
After a cutscene, you will both be teleported back to the Bastion. The achievement will pop when you talk to the survivor in the Bastion. Not just after watching the cutscene.
Taken from the 360 guide by FishyOutOfWater.