I actually do have extensive experience fighting this one ness, he's rather good so my advice may be of use to you.
He spotdodges a lot, so sometimes i land with a Dair to damage and knock him back just to annoy him. The U tilt works wonders on ness if he tries to aproach you wich the Dair or a way too easy to predict Bair on the front. Ness's Fair is indeed a pain, the only way to work aroud it, that i have found, is jump backwards at the same time Ness jumps at you, let the Fair run out, then attack with a Fair of your own.
You should try to grab at given oportunities, to add damage. Damage is more important than usual on this battle, because you really will need to send Ness out of the stage at the earliest time possible. After that, i fnt it very easy to edgeguard him, I use a Fsmash to kill his UpB recovery since I have nailed the timing with practice.
Be patient. Ness is very fast and will be able to combo you, but so long as he doesnt grab you, and you dont set yourself up for a Fsmash, he won't be able to kill you as easily as you can kill him. try to stay in the air with short hops, and If he likes to roll behind you to grab and Btrow, then predict it, and counter with a Klaw behind you, it'll grab him right away and you an do some more damage.
Fighting his Dair with your Uair is tricky business. I havent anaged to win that clash of attacks consistently, you're better off guesing were is he going to land and set up for a grab.
AH, almost forgot, Ness's dash attack can be a problem if you arent used to it. you have to keep the shield out for a bit longer than usual with other characters, before you can fortress or grab. If you let go too early you'll be popped up into the air and most likely comboed.
