This achievement is earned in mission 7 "A Crack in the Slab." After getting the time turner device (one of a number of Harry Potter-like things that show up in this chapter), you need to head to a small courtyard to the right of the main inner ballroom in the back-center of the house (the large multi-story room you would come to by going through the front door, straight ahead through the lobby and through the doors then down the short corridor). YOu can get to this courtyard by entering the large ballroom and heading to the right, or by cutting through the kitchen, grabbing the kitchen key from a cabinet (in the present time line), then exiting the kitchen through the small storage room in the back. The window in this room overlooks the courtyard.
To unlock the achievement, you need to destroy the two red canisters on the balcony scaffolding straight across from the kitchen storage window (in the past timeline), then switch to the present to discover it collapsed and climbable. It unlocked for me before I even climbed it.
There are a few guards in the past time line (and potentially some hounds in the near room in the present), so deal with them how you wish, or just ignore them to unlock the achievement.