I'm assuming you're always behind a trampoline, and you should be. Outside of a clutch misfire, Luigi shouldn't be able to punish whiffed trampoline attempts if he's on the other side of the trampoline when you place it.
So like this:
______Luigi________Trampoline________Pac-Man___________
I can't seem to beat Fair unless I predicted it waaaaay early, and fireballs hinder my game severely.
Yes I know I can SideB punish them, but then what? I now have a dot in front of me, but can still get punished for either picking it up or trying to charge fruit behind it. With a dot in front of me, I can't jab to stop an approach as I will grab the dot instead, and if I move away from it, then I am just giving Luigi space and giving Luigi a free heal (He can throw a fireball past it and eat it for free).
Don't leave a pellet on the ground unless you've got a trampoline up. Otherwise, you run into the problem you described...more or less. A close range pellet shield should be close enough for Pac-Man to eat without Luigi being able to punish even without a trampoline up. Regardless, always have a trampoline up IMO.
I am already placing a trampoline between us to stop run up grab, but he can just stand behind it and throw fireballs to wait it out. Admittedly, in this scenario I do get a few heals off or charge to a slightly higher fruit behind the dot.
If he chooses to wait out the trampoline, do what you said and pellet shield -> sit behind the pellet and charge to Apple or Key. As Meleebrawler said, you have Cherry, Strawberry, and Apple to throw at him from above. Trampoline lasts long enough for you to charge to Key. Once you get the Key, fireball becomes unsafe for him. Keep setting up trampolines and hover your finger over the special button. If he throws a fireball, you Key him while the trampoline prevents him from doing anything you can't react to.
When your trampoline is about to disappear, plant a hydrant so that the water will cover you setting a new one.
It hasn't happened to me yet, but if Luigi chooses to do absolutely nothing when you've got Key ready and trampoline up, go get the Key in your hand. At that point, constantly instant launch hydrants at him until he decides to do something about it.
This leads to another point: The only fruit to outright BEAT fireballs is key (And maybe bell, haven't tested it with Bell, but that's not exactly a counter zoning tool). I can't fight using only key
Bell loses its hitbox to Mario's fireballs, so I'm guessing it's the same deal with Luigi's. IDK why the devs didn't think Melon should beat them, but w/e.
You can fight with only Key. Predictable or not, it's still a 15% KO move that travels very quickly and cuts through fireballs.
I do admit I have more success keeping a key charged and fighting normally then punishing fireballs with key, but once key is gone my neutral game returns to a less than ideal state, and key doesn't give me much of an advantage state.
I disagree. We contain Luigi's neutral very well.
As for Fair... well do we have anything to outright beat it in neutral without relying on prediction?
When you see Luigi jump, throw a Bair at him. If you're standing right behind your trampoline, shield the Fair and attempt to punish him when he gets bounced up. Like Pacman9 said, our Fair is faster as well. If Luigi is jumping over the trampoline to get at you, pivot Fsmash him or Dtilt his landing if you want to be safe.
Honestly I feel like I must be missing something, because I don't understand how this MU is giving me so much trouble.
You have to play all out control mode and be patient in this MU. You said you don't fight Rosalina the way I commonly describe, so it must be against your natural tendency.
Also, notice you didn't say anything about Luigi's grab game. That means you're already doing the right things because that's usually how Luigi gets his mileage in this game. Like I said vs. Mario, you will rarely get grabbed in this MU if you don't want to. Not saying you won't eat another punish sometimes, but you're not getting grabbed.
Edit:
Tournament today and it should be pretty big. I'll show people what Pac-Man can do once again.