generally speaking, his killmoves are bthrow, uair and sweetspotted bair, just be aware of that
dsmash can kill near the ledge, since it has a weird trajectory, but it is laggy
fsmash can kill pretty early, but it is slow and unsafe
then the pkt2 kills, but they're mostly mindgames after you got juggled with pkt1
ness' dair is useless onstage so he can be juggled quite easily
if ness recovers near to the edge, you can try to just hit him out of his recovery, before he hits himself with his pk-thunder
if you play sonic, then you could up-b over him, so the springboard hits him while he is still trying to hit himself or after he did that, when pkt2 connects with anything (including projectiles like sonic's springboard), then pkt2's range gets halfed
sometimes you can even just jump into it yourself and tech it, you need that tech
his nair is the definition of a combobreaker if you're close enough to him, be aware of that move
pk-fire is quite laggy, try to punish that move
Ness' grabs are pretty fast and got a decent range while his dthrow is a great combo starter, try to not get grabbed (which is always easy to say)
Ness with ragemode + his bthrow is a biest
most general stuff i can think of right now