Mario, Sonic, ZSS, Rosalina and possibly the Star Fox characters.
Mario: do what @
Nu~
said and hide behind hydrants + trampolines. Pester Mario as he approaches with Fair, Bair, Utilt, pivot Fsmash, dtilt, and Usmash. He will beat you in straight up CQC, so attempt to control the ground and force him to approach in awkward ways. Don't be surprised if the MU goes to time. Your OOS game needs to be down pat for this MU btw, and you'll probably lose if you let Mario get away with stuff on your shield that isn't safe. Have up B and Nair OOS mastered.
Sonic: do similar to Mario except much more trampoline heavy. You need to have a trampoline out at nearly all times, or else Sonic will have his way with you in neutral. While he is fast, you should be able to shut down the spin dash game for the most part with fruit, jab, tilts, and trampoline. My advice is camping the ledge and focusing on punishing Sonic for attempting to get to you. I'm going to upload some vids of me vs. Brawlman in tournament recently. I haven't won against him yet in a set, but I still don't feel that the MU is any worse than 40:60. Most of our games are usually 1-3 hits away from going in the other player's favor.
ZSS: haven't played this one offline in a while, but hydrant and trampoline exacerbate her weak neutral. My best advice is to keep an eye on her and try to punish when she attempts to start something up in neutral. In general, she should be the initiator in the MU, lest you get setup time + whatever fruit you want. Do not get bullied by Flip Jump. Airdodge and force her into the footstool animation, or challenge it after the invincibility but before the kick comes out. Do not let ZSS players feel like they can get away with the move in neutral against you. Fair -> Key at high % is fairly reliable on ZSS as well.
Rosalina: this is a grueling MU that takes forever IMO. I recommend camping hard and forcing Rosalina to come to you by constantly Fair launching hydrants. Heal off of Gravitational Pull on reaction, but don't be so focused on healing that you miss opportunities to punish Rosalina. Use Dair to send Luma into tumble at any %, and don't be afraid to challenge Rosalina's Fair/Bair/Nair aerials with your own. Hydrant beats her Uair, so she shouldn't be able to juggle you easily. This is another MU where I'd recommend having a trampoline out at nearly all times to control her grounded game. Also, remember that fruit thrown from hand can't be picked back up. Get fruits in your hand if Rosalina is giving you time to set up. I think we lose the MU, but only 40:60.
Fox: similar to Sonic. Fox doesn't have direct KO options out of throws, but his Usmash is fear inducing. Luckily for you, Pac-Man has the perfect move to screw over his neutral. Always have a trampoline out and charge up to what you want / need. When Fox approaches, pester him. Unlike most reflectors, Fox's is actually a pretty solid commitment unless he actually reflects something. Because of this, he shouldn't be throwing it out as liberally as some other characters would. Use this to your advantage. In CQC, Fox's jab is a beast, but I found that our own tilts and jab are good for fighting back at any distance that isn't point blank. Use orange when edgeguarding to hit Fox out of side B, and I recommend jumping out there to put pressure on Fox. You need to harass him as much as possible when he's offstage, so don't waste time charging when you could be setting a trap or jumping offstage to hit him.
Falco: haven't played this MU in tournament, but I think our zoning is powerful enough to keep Falco at bay. He doesn't have much mobility, and he can't camp us really because of hydrant + power pellet. I recommend doing typical zoning with hydrant/trampoline and charging to what you want for a setup. I'm speaking mostly from the perspective of the opponent trying to force a conflict, because that's what smart people will do against Pac-Man. Pester Falco like any other opponent when he attempts to cross your trampoline, and always be mindful of his reflector when you're using your fire hydrant. TBH, I rarely launch it at him in the MU because it's very much an obstacle to him if you don't. His lasers take a while to launch it, reflector takes 3 shots IIRC, otherwise he's knocking it over with tilts or something. IDK, play the MU like Luigi, except mind the reflector + much better vertical mobility. Also, if you read a reflector toss, try rolling at him -> punish, powershield to running Nair, or PS to dash grab.