When the mission begins, sneak towards the docks and to the right to trigger a conversation. You will learn that Niculescu is planning to kill Mara, and the latter is held in the main bedroom. Head back to your starting position and get out your taser. Aim it at the guard opposite. Zap him and set him aflame, then turn around and zoom in on the second guy with the sniper rifle. Sift his head.