[ Register or Signin to view external links. ]

This achievement can be earned during Mission 7, A Crack in the Slab.

After obtaining the Timepiece from The Outsider, leave the room via time travel, similar to how you would for the "Under the Table" achievement, and remain in the past (with the manor fully restored). Follow the same path through the corridor and through the kitchen and out into the courtyard as you did for the "Flooded Basement" achievement. Take care of the hound here and move yourself into the alcove on either side of the yard.

Time travel back to the past; there will be two guards present in the area: one in the courtyard itself and another on the balcony. After they have finished their dialogue, eliminate the lone guard and turn your attention to the now-empty balcony. Use your crossbow to shoot the two support beams of the balcony. After doing this, time travel back to the present and you will notice the balcony has collapsed. Climb the rubble in order to reach the second-story floor and the achievement will unlock mid-climb.