Honestly, choosing ness for that matchup is tricky. Grabbing out of shield is their way to get you. Only use aerials you know you can angle away from their grab range if shielded. Idk if anyone uses pk jump (i havent paid attention to tournies in 6 months) but it's useful for things like this. You can space it really well and have time to react if they aim for your landing. to get out of the grab i just spam the c stick and taunt and is doesn't last long ( this may work more on inexperienced IC's rather than on someone that's gottem down packed) but usually in the sets i've been playing recently (playing lotsa of different people for a return to the competitive scene) IC's simply need to be kept out of grab range, meaning you should pick stages you can have the running advantage in (i.e. battlefield, lylat, ps1) because in places like fd and often SV, there's almost always one place and one way to land when you're out of jumps. Grab armor isn't something to be scoffed at, so just know their grab statistics (range, frame hit, ending lag) and make a list of options before you play. Anything else is usually obvious, getting them offstage, separation, etc. but what helps is thinking of exactly what ways you can get them offstage without them f***ing you over and grabbing you. You can't truly play campy with ness unfortunately, and you can't be agressive without screwing up, so you've just got to keep a healthy pressure to the point where they don't think they can create a chance to grab you. Whether it's because you're doing strings like (shfair moving away -> shorthop+airdodge) or (backwards pkjump off the edge -> magnet to the ledge), even the cute little arc of thunder we defend with on the ledge, so long as you find a way to land that doesn't put you in their range. I know this sounds basic, but its importance is understated.
Like, even making notes charts and graphs formulating options, patterns, and lines of attacks are useful, because work out of the game is a bit more important than actually playing endless matches. You've got to always have a plan and some backup plans.