Throw Ness off the level then jab or tilt or whatever with like 20 characters or so.
Then when he double jump backwards because he has nowhere to go in the world (assuming he didn't get hit by it) just space a Bair or whatever your characters' ranged aerial move is in such a way that he can't Fair through it. Try to stay close to the stage, so you can land and throw him back off if he airdodges through with good timing.
Repeat until dead.
That's the gist of it.
It's really gay, mostly effective with good characters. It's not always Bair and Jab, but those are common ones. Sheik you're more likely to F-tilt, Fox is probably going to spam aerials, etc.
Other variations of this include just spacing Bair (or other move) when he's free falling in such a way that you beat his Fair, and can land and punish or come down with another move and combo him if he air-dodges through the initial Bair. Because Ness can't really do much once you rob him of his ability to hit you with Fair during recovery.