The moment Ness jumps at a range you can reach him, run up shield beats nearly everything he can do bar a tomahawk grab (which takes a while due Ness's floatiness). Avoid tossing Pikmin when he's thrown out a PK Fire, it'll burst and cause you to be unable to pass a part of the stage (I mean, you can try and pass it with dual stick SDI, but risking getting grabbed by Ness is generally not worth it) unless platforms are in play. Once you get a juggle started, stick under him like glue. He has bad options to get down with, of which I'd imagine only Dair and b-reversed/wavebounced Magnet to hinder juggles. Dair only works if you don't space your Up Airs, though, and Magnet is just for momentum shenanigans in this scenario. Unless they're a literal living god, edgeguarding Ness is fairly easy. Clip his double jump (while respecting rising aerials from him), it forces him to use his horrendous up b. You can try to "eat" the PK Thunder, but if you mess up, you either die or get in a reverse edgeguarding scenario. Just wait for him to get to you and Bair him repeatedly. If he's trying to aim for a sweetspot, it takes half a century for him to grab the ledge while spinning at it, so you can Down Smash it pretty easily, which will probably kill him because of the terrifying angle it sends you at. Ringing
@steelguttey since he plays Boiko every now and then and can probably provide better insight.
Against Marth, DD right around his tipper range (specifically his Dtilt and Fsmash) and force him to swing early. If he jumps, beware of double Fair out of his SH as the second Fair beats anything you try to do OoS, wait out the second Fair and punish it. Either character can get juggled pretty hard, so if you're caught in one, aiming for a platform (with a good tech) or attempting to b-reverse whistle to mess the Marth up are about the only things you can do if the Marth is on point. Pikmin Toss is fairly useless in this MU, since he can do just about anything and ignore them. Purples are godlike as usual when tossed, even more so in this MU, though. Smart use of F-smash to make sure he respect your space and doesn't just dash in > grab you is also kind of needed, as getting grabbed tends to lead to juggles. Edgeguarding is brutal on both ends. Poor sweetspots can be CC Down Smashed. If you're covering the ledge as you should and force him to recover on stage, you can ledgedash to grab and throw him back off or ledge drop > DJ > Fair to knock him off stage at the other side hopefully. For off stage edgeguards, you can try them, but respect rising Fair, as it beats any poorly timed attempt to gimp him. Fair is generally the safer option (as it comes out earlier), but Bair can gimp him at early percentages as it's another move that sends at at bad recovery angle.
Oh yeah, F-smash does a great job at edgeguarding both of them, or at least cut off a low recovery. F-smashed Pikmin can even "eat" PK Thunder, which is pretty great if you can manage it. Don't get too predictable with it though.