Can be done on any difficulty. For the barge level you start off without a gun, so stealth is required here. You also have to provide sniper cover for Vesper, being careful not to shoot her, not to let her get killed, or get killed yourself. The Venice level has a tricky part where you get trapped in a courtyard and get attacked by a ton of guards at once. Not too hard on lower difficulties, but rather brutal on 007 difficulty.