Near the end of the Act 1 after you cross the rushing water, you will be in a cave with AJ. After some dialogue you will get a choice between saying you trust AJ to make the right choice when it comes to killing someone, or that he isn't ready yet. (You get an achievement for either choice so you may want to make a backup save during this scene)
For this achievement, say that you don't think he is ready yet. Now continue playing into Act 2. There is a scene on a bridge then you climb a cliff - the achievement pops after that scene.
Choosing the other option (you trust AJ to make that choice) will unlock the Goodbye achievement instead.