Falcon: Jab him when he approaches. If he dashes back, assume that he's coming back and jab again. Don't jab blindly, and expect him to roll behind you at any time. If he tries to arial you, you can OP it with a jab. He'll get mad pissed eventually. He's fast, but you have a big sword. Avoid grabbing him (unless you're the **** at TCing and have lots of patience, because you can grab him six times and he'll still be around 20%, while he can 0-death you at any time). Bottom line: when he's coming at you, swing at him BUT DO NOT FSMASH UNLESS YOU KNOW IT WILL CONNECT. He's also extremely easy to keep offstage.
Sheik: Stay away from her hitboxes. Pick a ledge or a platform, and if you're above her, avoid uairs. She's easy to combo, so 0-deaths aren't very difficult (at least as much as they are with, say, Falco or Falcon). If you're on the ledge and she's recovering over you (to get on stage), WL past her and fsmash (or utilt/usmash for slashiness if she's on a platform). I don't know a lot about this MU, but it's pretty overblown and all you need to do is space very well, don't **** up on combos, and avoid needles. Shiek starts to win when you walk into her grabs and tilts. One way to space/zone well is to get near her and start SELECTIVELY fairing and nairing with the occasional uair or tilt/jab, using both full and short hops. NEVER start jumping AT her with sh double fairs or whatever; make sure to only acts as a wall of sword so that she may not pass/ do what she wants. Keep her guessing. Sheik doesn't have the same control over Marth as he can have over her, and as I said, she begins to win when the Marth can't play a good defense/space game. As a side note, well-timed ccing/light shield use can save you from getting ****ed up. One ftilt is all it takes with this *****.
Falco: I need help with this one as well, but for the most part, once you get past the lasers, he's like a ******** version of Fox. How you do that is up to you, but whatever you do, don't stand there and shield as he hops toward you. Learn to shield drop consistently and you can spend more time hiding on platforms. Learn to switch to lightshield as often as possible. Shield pressure is nothing to a light shield; you just slide away/off the edge, or the Falco gets tired and grabs. Countering is a GREAT strategy, but you need to read your opponents in order to pull off successful and consistent counters. There is no reason why you should get hit by Falco when he's on the ledge. Predict his Ledge behaviors and you'll end up killing him when he tries a dair or double laser.
Jiggs: Don't walk into bairs or other arials. Keep swingin' and stay far away from above her at all costs. With good reaction time (on your part), this can be one of Jiggs' worst MUs, because there isn't a single thing that she can do that can't be OP'd by your sword. Since she's pretty impossible to combo, it's mostly just about racking up damage and remaining patient. She's really light, so grabs are great on her. Randomly placed f/bthrows can lead directly into a tipper (or other maneuver) if she ASDI's poorly. Also, surprise her with a followup WD grab after an fthrow sometimes.
Peach: Pretty easy. Rack up damage with fthrow chains (until she grows wise), learn to catch turnips, and fair/nair her offstage and upb/dair her while she's over the edge. A Marth with consistent combos can be a very scary thing to a Peach. Random fthrow fsmashes are too good. Also, if you crouch your fsmash while peach is stunned from the grab, you'll have a higher chance of tipper. If she DIs, you may be able to WD fsmash. Pay attention to her float habits and smack her out of the air when she tries her dumb ****. Once her % is too high for consecutive hits, it's all about how good your defense is. Keep fairing and uairing, and look for any utilt possibilities. Uthrow kill if you have to.
Fox: The favorite of many Marths. He's like a little toy that you can throw around, and you'll win if you don't **** up. A missed l-cancel, getting stuck in shine, or a bad recovery job can all result in the death of the Fox, so get good at punishing and this MU is extremely simple.
Marth: Don't get hit.
Additional stuff:
Lightshield often
Learn ALL of your oos options
Learn to counter properly
Observe how Marths such as M2K can systematically eliminate opponents' options
DD whenever possible, and WL/WD more than you run. You can do any attack/lightsheid while sliding, and running around limits your options and makes you easier to read.
Keep swingin' and stay consistent
Be smart
With floaties at high %, be really gay and grab them only to knee them into a higher %. When they break away, try for a regrab and do this until you can throw them through the ceiling or kill them with a fair.
That's all I got.