Hi there.~
So, it's definitely around 60:40, but I don't like to focus on ratios so use whatever you want, that's just an approximation.
You need to force Ness to approach no matter what. Fsmash is not a good call for Ness because while it does reflect, it's slow. If I remember this right, Ness' jab can stop arrows, and he can pick up bombs with Nair pretty easily.
Don't let Fair walls happen, at all and don't try to hang on hookshot for very long, PK Fire is a great move to use against it.
Watch out for PK Thunder mindgames, if you get caught in the tail it's not easy to get out. If you want to gimp Ness, honestly, I'd either use an arrow if you have good timing, or just jump in the way of the tail haha. If Ness has been doing PK Thunder for more than half a second don't even try to touch him, you'll get hit and the knockback is tough to deal with. If you're risky and he's kind of far away, you can probably let yourself hit him (if you're at a LOW percent) and that will shorten the range, and he'll die. I don't really suggest that though...
Don't let Ness grab. He's got good grabs. Fthrow does 11%, Uthrow's 10%, Dthrow sucks but is good with a PK Fire combo and then you've got Bthrow which kills everything in sight.
I guess that's all I've got. Watch out for fast aerials, throws and the dair spikes. Your best option is to just camp and spam like crazy.