At the end of the game when you are back in the Spire to get your revenge against Lucien, make sure that you save and continue the game. Once done, face and kill Lucien. Select one of the three choices Theresea presents you with. After you unlock the achievement, immediately reset and restart the Xbox 360. Choose to continue the game to get the next achievement until all three have been selected and unlocked. You can reset the Xbox 360 a fourth time to select the choice you really wanted to begin with afterwards.