This achievement cannot be missed. After Completing memory 6, you will be kicked out of the animus. proceed through the dialogue and follow Lucy through and you will find that you can do all off Ezio's moves, but as Desmond. After playing in the warehouse, Desmond will be pulled into a new place, based on his memories alone, no animus. Since the game is still new, I don't want to spoil, but proceed through the sequence, and once you finish, you will wake up. Re enter the animus to unlock the achievement.