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It's nice to see that other Mewtwos see that Abadango doesn't know the Diddy MU. It's what was pretty obvious to me the moment I saw him play against one. He didn't play neutral correctly, didn't combo him correctly, didn't edgeguard correctly... it was kinda weird to watch him play that MU. Mewtwo outspaces Diddy with dtilts pretty badly, so dtilt setups from Diddy aren't that scary. Nair edgeguards Diddy almost for free and you can camp him with Shadow Balls. Banana tosses aren't too threatening, though Diddy has a huge space he can cover if he has a banana on the ground and can jump around with fairs or dash in with dashgrabs etc when Mewtwo's cornered or wants to get back to the ground. Since Mewtwo doesn't have a really quick aerial it often has to rely on airdodges to get out of stuff which can be predicted.
Luigi is a pretty decent character, it's just that some good characters hold him back a bit. Namely Sheik, Mewtwo and probably Rosalina and Greninja (of course there are more MUs where he can struggle but those aren't so common). Though that doesn't mean that he can't win in these MUs. They might just be -1 for Luigi.
He does have some nice MUs against a few characters, like Diddy and Mario, probably Fox as well. Not sure about Pikachu (because if he plays safe and campy it's hard to appraoch him). Diddy and Mario get combo'd for free. Fireballs stop Diddy from sideBing or banana tossing and make him shield. It's pretty tough. While Luigi outranges Mario most of the time, is stronger, and somehow it's really difficult for Mario to get back to the ground if he got hit upwards. He can also get gimped for free if he has to use his upB. Fox is kinda annoying to deal with but once you get in you can build damage from that and push your advantage and his disadvantage. If he ever has to use upB it can be a free gimp.
Though while he's a decent choice he can struggle against safe zoning/camping playstyles/characters, if the opponent chooses to do so. Or at least it can be pretty frustrating to deal with.
His runspeed is kinda poor, so often he might try to use Fireball to help him approach if he has to, which isn't too safe anymore thanks to the added endlag frames (and needles go through them). Jumping above projectiles/approaches doesn't help much since he's so immobile in the air and falls really slowly. Oftentimes he has to try rolling behind his opponent which can easily be countered by using retreating aerials (Sheiks/Grens landing fair) or similar things like that.
His recovery can also be really stressful and frustrating. It's probably a big reason why not many people choose to use him. With no other character do you have to get worn out and be unsure about being able to mash correctly to even make it back.
Luigis sideB is so easy to intercept that you oftentimes can't even use it or you'll get hit away again or get spiked out of the endinglag of it. Even going pretty low is unsafe against some characters. So you oftentimes have to rely on your mashing skills. Without being able to use the jumpless cyclone Luigi gets much worse overall since he's so easy to gimp and edgeguard without it. And even if you can mash... there are times where you just fail and it's pretty frustrating.
Luigi vs Sheik MU:
A Sheik that knows what she's doing can easily get fairs or tilts or grabs on Luigi which can all lead to fair strings to get him offstage. On the ledge he has a hard time to get back, because if you ever use your doublejump and get hit out of it you might die, so it's a super risky option to ledgedrop doublejump, even if you cyclone or airdodge. So your options are even more limited. Cyclone mixups can be shielded and punished for free (even with smashes) and getup or roll can even be reacted to. Jumping can work for a short time until you realize you have to safely get back down again against Sheik.
Like I said before, needles go through Fireballs. If you have a bit % racked up it's even really bad to trade since you'll get hit up and have to land again. Neutral is sooo difficult against Sheik for Luigi, since jumping (besides sh fair) is just so bad. Sheiks groundspeed and dashgrab/attack are so fast, needles threaten, her safe retreating fair is basically unpunishable and sets up for fair strings/combos that put you offstage or at the ledge (terrible positions).
And if you ever lose your doublejump and get hit offstage you're most of the time a goner if the Sheik knows what she's doing. Ever have to use sideB? -> free bouncing fish. No matter how far away from the stage you are, no matter how low you go. The only time when you can use it is if you're suuper high up, or the Sheik hit you so far away that she's still trying to reach the other side of the stage to edgeguard you... every other time she can safely bouncing fish every sideB attempt. With no risk, it kills pretty early, and since Luigi might not have any other option he'd have to use it again. Only thing you can do is hope for a misfire.
Comboing her at low % also got more difficult with her new weight"nerf". The old dthrow -> charged usmash -> turnaround regrab -> dthrow -> fair -> fair/bair/uair (-> maybe another regrab -> fair -> uair) doesn't work anymore, so he lost a lot through that. Now it's super difficult to time the usmash charge to attempt a turnaround regrab. Most of the time you only get like 20 % ouf of it instead of the free 40-50.
And even though her fair range got nerfed, it still outranges Luigis fair nicely enough. So oftentimes I try to go for bairs to be able to challenge anything she throws out. But if she's patient she can just wait (mostly crouched). Bair has enough endinglag in the air to make it a really risky choice (especially since Sheik gets so much out of any hit if she manages to catch you), and even if you land with it it's easily punished with an ftilt, dtilt (both which can set up for free kills at high % or fair strings at lower %) or dashgrab/attack. You also have to get pretty close to her to hit her so she might try to just fair you before you use your bair.
Now compare that to.. Mario for example. In neutral he can freely jump around if he wants to. Is pretty safe, can threaten Sheik with bairs, nairs and uairs which are super fast and safe and can setup for more combos. Can recover much better (oftentimes his airspeed alone will drag him back so he doesn't even need his doublejump most of the time, granting him much more options). His combogame in this MU got better than Luigis after Sheiks weightchange imo (before that it was probably about even). He falls faster, so overall being able to jump around and be so mobile makes him get grabs much more easily to start combos. He isn't threatened as much by needles (since his disadvantage isn't as bad, plus he has cape which can make Sheik doublethink to try to punish a landing with them, not to mention it stalls if he uses it above the ground). And if he gets hit he isn't in a terrible position. The MU is SO MUCH easier, though at this point it's probably kinda safe to assume that Mario could win it +1. Even if you're behind, since Marios recovery isn't as bad as Luigis, he can live pretty long to make up for that and get some early rage usmash KOs.
For Luigi it's really just that he has troubles in neutral, his "normal" disadvantage is kinda risky, and him being offstage is terrible. Last time I played against our Sheik he could just edgeguard me a lot resulting in KOs at probably the 50-120 % range most of the time (because I had lost my doublejump (mostly while trying to get back onstage from the ledge or while being edgeguarded)). That just doesn't happen to other characters because their recovery isn't terrible against Sheik, so they can try to come back since she now has troubles KOing.
So to sum up.. as Luigi you have to be pretty dedicated, be able to (/ know how to) mash, and live with a few struggling MUs and some annoying strategies against him that non-slow characters don't have to deal with.
Imo people could use him as a CP more often, but they'd need to use him well, of course. I think his MUs with Diddy and Mario are +1 (for Luigi).
