PK-ow!
Smash Lord
Apparently M2K plays them. ._.If you have only wifi and nobody else, can't go out to tournaments, then you're better off using wifi. CPU's never helped anybody get better.
But he's a, err, 'statistical outlier', so I guess I'm just spamming.
The real reason this matchup sucks is because Ness has a vanishingly poor ground game against Marth... and that's supposed to be the other guy's weak point. You go in the air, Marth loves fighting things in the air. He only dislikes fighting downward - and... Ness can't really do much there without platforms.
What cinches this for me is that the Marth can not know the matchup and still do well. Not completely green, but just stay clear of PKT2s and respect the offstage ability Ness has, and he'll do better than alright.
If he is doing dumb autopilot things though, aerials everywhere, get in there. And try to stay close to him for as long as you can, dealing damage. Choose weaker attacks over stronger ones, even.