This trophy is related to main mission nine and CAN be missed.
Make your way to the very top of the lighthouse. If you have a low overall chaos rating, eliminate Havelock next to the fire place, and then grab the key on the table nearby. Head to the room Emily is in to rescue her, end this mission and game and unlock this trophy.
If you have a high overall chaos rating, trace the spatters of blood on the ground to the walkway outside. Shoot Havelock from a distance, and then rush towards Emily. Press to rescue her from falling and unlock this trophy.
Sometimes, Emily will fall straight to her death with Havelock when you shoot him, making this trophy missable. If that happens to you, load your most recent autosave. The game should save automatically just before you approach Havelock on the walkway.