This achievement requires you to play through the entire game a second time. Some people are having trouble getting this to unlock, so here's a few things to keep in mind:
Do not use Chapter Select at any time during either of the playthroughs! This can apparently void the achievement causing you to not only have to replay it again, but an additional time after that!
Let the credits play out during both playthroughs. You may be tempted to skip them, but this can void the achievement and you don't want to undo all the work you just did. They're not even that long, so just let them play.
I would advise playing through both times in one sitting, one right after another. It isn't popping for some people who come back to the game after a while, even if they don't use the Chapter Select. It's only 4-5 hours combined, so just block out some time to make sure you don't run into any problems. This is what I did and everything popped without any issues.
Hope these tips help eliminate any future headaches for people!