Simple, finish the main story and confront The Father once more.
Attention: You can choose two endings. The achievement unlocked for me, after I replayed the last mission and had chosen the other decision. (After the credits, you can continue your game and the last mission is still available)
Edit: ComboChrist gave me the info, that the last mission is only replayable if you choose to walk away the first time.
Edit 2: Seems, the last update screwed things up a little. Some player report, that after the "walk away" option the first time they're not able to replay the mission. As far as I am aware, both, my friend and I, got the achievement after choosing "walk away" first time and "stay" the second time. "Stay" is the true ending (you'll notice this by the change of your background in campaign menu) We finished the game on his save in co-op and I completed it on my save alone.