After the cutscene head over to the right. Follow the tunnel. After Arno jumps the small barrier, turn to the left. Follow the corridor while sneaking. Look to the right to confirm there is no one watching you then head down it to the left. Run over the lip of the lower area and land on the wooden beams stretching across the room.
You can easily kill them using 2 Phantom Blades and an air assassination. You can also throw a Cherry Bomb toward the far corner of the room, away from the tunnel or beyond the barrier on the same wall as the opening. Either way, head to North.
Follow the path. When it turn to the left, start running. Slide underneath the grating that would otherwise block your path. You will see La Touche who will run through the door in front of you can lock it. Go to the right and head through the door there. Turn to the left and take the ladder against the wall up. This will lead you into the main chamber where your target is.
Turn to the right and slide through the open gap into the next section. Climb up the ladder on your left to get to the next level. You can also use the Phantom Blade on the nearby guards for an easy completion of an optional objective.
Use Eagle Vision to spot your target across the way from you. Look to the right and deal with the nearby rifleman. Now just make your way over to the right to get into position to attack your target. Just throw a Cherry Bomb to give you plenty of time to where you need to.