Only by combining forces can they beat him. But they've got to be willing to loose to win.
That's not going to happen though is it.
Basically, you've got to anticipate him, so get someone up the road who's strong enough to win then use your other riders to mark Sagan so heavily that he can't get across. I think the 2011 season with Cancellara could be indicative, he [Cancellara] was so strong that it was obvious you had to out think him and his team, rather than over power him. So at Flanders QuickStep got Chavanel up the road early, then when Cancellara went, he was there to sit on him and not work. At Roubaix, Garmin got Vansummeren up the road, then Hushovd sat on Cancellara and wouldn't do a tap until it was too late.