I think fireballs are pretty terrible and don't force an exploitable response from the opponent >__>
vs. Sheik:
Fight grounded sheik by spacing with a combination of spaced ftilt, wd approach sh bairs, empty sh to wavelands, empty sh to attack, and platform wavelands, if they're patient and actually bait things correctly. If they're a mediocre sheik who's staying grounded you can probably just approach with attacks and **** them when they aren't prepared for your CC dsmashes.
If they're an aerial sheik then you can do something like space sh approaching wd bair at the spot where they land. (Fair beats luigi's bair straight up, but bair has longer range still, so if you aim it at the spot she lands she's at least threatened and has to shield or something.) You also have to take advantage of the (short) window of time when they are jumping to force them to do something other than sh ac aerial spam. If you predict her jump you can try to hit her as she's jumping or get under and pressure with sh uairs or something.
If she's at KO percent and you are not you can space your fair to trade with hers, but otherwise this isn't a good idea.
Basically I think direct approaches are terrible in this matchup except when you've used your weird crap to force her to stop baiting your linear approaches, and then you can mix them in. Or when you have a straight up read.
vs Falco:
I have no clue.
vs Marth: If they DD a lot then play it like Falcon. If they space aerials a lot, fake approaches and approach with shield to condition them to continue spamming aerials, then time an attack to beat their aerial, either dsmash or upward angled ftilt, depending on how they are doing the aerial. Marth's aerials actually have a lot more vulnerable time than other characters'. Dtilt is really good vs. Luigi, but it is punishable still. You can get in attack in during the lag if you predict the timing.
When he hits you with something, like an aerial or a dtilt, CC it or something and either dsmash (if there's enough time) or just get out of there so he can't grab you. Grab is the best thing he can get on you, since it leads to you being in the air and never getting down/getting ken comboed.
If they're trying to be maximally gay and dtilt, shield, grab, and CC a lot, then beats me. Just outplay them. Nobody plays this style properly though from what I've seen.
Luigi's combos on Marth off dthrow are pretty decent if Marth doesn't DI. If Marth starts DIing those, mix in fthrow combos.
Edgeguarding Marth is really, really easy if they suck at recovering, and kind of hard if they're good at it. If they're bad at it just use ledge invincibility and hit them as they do some obvious recovery attempt. If they're good at it then you kind of have to predict their recovery timing and edgehog at the right time. Or, you can try to space a sh dair around their up B hitbox and hit their sweetspot attempt, but I think this depends on how much they hug the stage/how they are curling their up B.
I'm not sure if lightshield edgehogging works; if it does then it'd be really good cause of shoryuken.