Yeah
If I had to list the top three characters that give me trouble as Luigi, it would go
Metaknight
Metaknight
Metaknight
I'm convinced Luigi can handle everybody, but Metaknight is by far Luigi's hardest match-up. All my advice needs to be taken skeptically- as far as I know no one has a Luigi yet that's proven truly effective methods of countering a good(important modifier) Meta-knight. But I'll try to help
-Meta-knight's Mach Tornado makes a V shape around metaknight, which leaves an open spot above MK where a Dair will always reliably hit him. None of Luigi's ground moves outprioritizes the mach tornado, but fireballs may do something. You may get some of Luigi's aerials to outprioritize however, depending.
-Try a different method than you are probably used to for spacing. Instead of short hopping aerials toward MK always, try short-hopping backwards with a fireball followed by a nair or land a tornado and rush MK. A lot of Metaknights and Luigis are both used to simply getting in close to an opponent, ie. a lot of shorthop aerial approachs right up close. Problem is here, luigi loses out to MK doing this. If you move back while he moves forward, you keep a close, but separated spacing which you can work with to get an opening, and MK ends up taking more risks to hit you.
-Your uair with good timing will beat MK's Dair when he's above you.
-Everytime any MK ends any of his B moves in the air, he gets a general stun fall. He also doesn't fall too fast. So turn those opportunities into a guaranteed firejumppunch.
-See what you can do to keep yourself on the ground when fighting MK more- try throwing a few fireballs, then approach with a tornado. Downsmash is pretty good, too- Dash to spotdodge to downsmash or grab, or do a full tilt walk to downsmash(it slides nicely.
-If things get spaced where MK is right in front of me for whatever reason, like I just landed an aerial in front of him, Luigi's 3x jab is a nice, safe, effective move that rarely gets you punished. If he's behind you or something like that, utilt is a deceptively fast, good ranged move.
-Mix up the tornado(falling only), nair, and airdodge to avoid ever being juggled. As long as you don't rely just on one, it should work.
-When you approach with short hop aerials, don't follow the same pattern every time- mindgame it up immensely. One time- you fair to fast falled sex kick. Next time, you jump- don't do anything on the rise, then fast fall a bair, etc.
-If he starts spamming downsmash and you shield it or dodge it somehow, either just let it slide you away or roll away.
-MK's love shuttle loop edgeguarding people. I've found that the safest way to avoid being edgeguarded as Luigi is to charge up over-B and aim to sweetspot the edge. It comes in very fast, level with the stage, with a hitbox and a strong hit, and you'll snap right to the ledge. And if not, you're not screwed anyway. I found the alternatives put me in places MK would KO with the shuttle loop.
-Need an out of shield option? Don't mess around! If you just shielded some attack of MKs and looking to retaliate, 9 times out of ten you can just jump out of shield short hop nair and stick your foot in his face. With a sweetspotted nair on a lightweight like Metaknight, it can even KO.
Finally
this isn't a matchup that seems to be in your favor, in terms of being out prioritized in every way in the air. However, you have a chance to make up for it with Luigi's heavier weight and much better KO power- especially vertically. 44% firejumppunch will KO MK, 72% up-angled Fsmash(which is lightning fast and repeatably spammable too), 94% upsmash. On Final destination. Counterpick low ceiling courses whenever possible. The upsmash KO % can go down a healthy amount if you do a running upsmash, you'll slide a good distance with time to charge- it's both an excellent way to land a smash on MK because it is fast and hard to predict and the upsmash flings luigi's giant head in a large arc, and has satisfyingly high KO strength. Sweetspotted nair will start killing at about 104%, and for side KOs just based on low weight you'll get great results out or backthrow too.
You have the possibility of quickly making up for the fact that MK hits you more often than you hit him when he starts dieing at half the % you do.
Right now, none of the MKs I normally fight don't win much, but I recognize it as Luigi's hardest matchup and there are lots of MKs out there that would beat me, I'm sure. Hopefully I'm somewhat helpful with what I got
