*uses my moms amazing Romanian cooking to lure Simna to NoVa so I can learn from him AND he gets to play Mow AND Mew2king AND chudat."
*drives up to NoVa*
as for Marth...if you can make an opening for a fair, but he Crouches it into a poke...maybe you can try it a couple more times to make him think you'll keep doing it and then dash at him like you were going to DJC fair again, but then just do a dash attack which will pop him up and then do what you wish [I haven't had enough experience against GOOD Marth players with Ness so I'm witholding a thorough description]
...also, don't try upthrow-PKT, he's kinda floaty and he he has really good stun-recovery speed, so he'll easily dodge it and come back and punish you before you can swing it around...unless you predict the dodge and you already start swinging it in his direction, but even then it's not really practical.
stick to uair-bair combos...I don't think there are any great ways to combo Marth beyond a 2 or maybe 3 hit combo...just...rack up damage and go for a killing blow. Also, I think that maybe [huuuge maybe since I've only tried this a couple times] if you can space yourself correctly and bait a f-smash out of marth, if you're just out of range of his sword, his swing will put him in range for the bat [so long as the sword has mostly swung all the way as the bat is coming out, otherwise, maybe they'll clank?]
also, and I'm sure I've stressed this before, if you haven't gotten this down already, master the full wavedashes. it'll save your *** against Marth's sword. also dashing into full wavedashes [actually dash->WD is fine too] makes Ness extremely fast and you can mix it up a whole bunch between dashdancing-->full-WD-->DJC'ed aerial [or u/f-tilt, maybe grab?]...the good thing about starting out with the dashdance is that WDing from it allows you to either DJC fair or bair [like in the Greta vid when he's against Yoshi.....is that..Yannick? iunno...but yeah, just watch that and you'll see what I mean]
the only good way to land PKfire is to make Marth miss first and hit him with it while he's attacking, the fact that PKfire dash-cancels helps too. or if you can manage to make him follow you into the air [as you float backward] and then nail him...I don't know if any of Marth's aerials can cancel out PKfire
I'm not that experienced of a Ness player when it comes to going against really good people, so if any of what I said sounds impractical to those who are probably better than me, please lemme know or give some additional pointers
thanks