This and "You and Me" achievements suck so bad... cause the achievement is bound on the choice of if you trust AJ to make the tough decisions or not. But here's the kicker,
you don't get the achievement till after having climbed up the rocks after the bridge scene which is a long time after the trust choiceFor this one, you need to trust AJ with tough decisions. Which will end up with Tenn
If you want to get this and "You and Me" in one playthrough then here's the how:
After crossing the river in the cave, the game will autosave, once the scene loads in and the save icon disappears, quit out into the main menu and make a copy of your current save then on that copied save, do the path you'd rather not do (i.e not trusting AJ which gives the "You and Me" achievement) then once you got that, go back to your main save and do that all over again but this time going for the opposite one, which gives the "Goodbye" achievement.