Let the guard pass, then open one of the windows to sneak behind him for the Knife kill. Jump from the window to land on the nearby roof. Even consider backtracking to charge it, just to be sure. Even if you don't think you need to, stop at every charge station and fill it up.
Tip: Keep the Laser Cutter charged at all times. Use the Laser Cutter to cut a hole in the fence and sneak through. Head back the way you came and up the stairs to find a place you can charge the battery. Go through the door in front of you and turn left to find the battery you'll need to operate the Laser Cutter. Continue along until you drop onto a roof, then climb down through the hatch.
Snag the letter off the table and then get back to the ledge. When you resume, jump the railing, maneuver along the ledge and climb into the first window on the left.
