With Luigi and Link banned, GW is probably your biggest threat. Only matchup that I can think of that isn't either really close to even, or in Ness's favor. And since I play the other end of the matchup more often, my best advice is to never, ever let him get you off the edge. (harder than it sounds against someone sufficiently good) Abuse his lag on 3/5 of his aerials (non-cancellable), bad shield, bad dodge, and bad roll whenever possible. Again, harder than it sounds if they are really good...
Sorry this advice is late anyways, but still good for future reference.. ^_^