work on your spacing. Don't dash dance into Fsmashes, you're supposed to be outside his range and bait the attack with it as mixup in your flow so you don't get counter ***** all day or grabbed. Also, at his level, baiting is fairly effective so you can most likely jump in just outside his range and bait the fsmash because obviously he doesnt know what short hop fair is, and then you can jump out, he whiffs, you come in with bair. Since he isn't that great he's probably not gonna drop your stock off grab though so you just need to get better at spacing especially with dash dance, also be speedier when coming in with nair and lasers, vary it. IF A MARTH IS SPAMMING FSMASHES SHIELD THEM AND WAVESHIELD INTO GRAB/AERIALS (depending on spacing). Especially if they are kinda noobish they will not have a response to this, and may feel like you took away their fsmash option and not use it really, or, they will just keep doing it because it works every once in a while, and you can just bait the fsmash by dashing into a shield or whatever else.
Also, really work on doing reads when it comes to your teching options. Yes, when between marth and the edge you can get stuck eating fsmashes on roll into marth because your options in general suck when spaced and timed right, but away from edge, you need to learn to read his timing and position and what he thinks you will do, OR if he IS NOT READING YOU because he is noobish and going off of spam and little thought, then just read what you think he will do because he isnt gonna mix it up based off of your patterns.
He also doesn't know how to space aerials so you can easily beat him with bair. abuse it until he learns to mix up his approach and control the skies. if you jump over his *** he is probably just gonna try to fsmash, you can cross him up with bair or just come at him backward all the time and jump in and out, play the game to be most effective at his skill level and abuse what works.
also work on using grabs to set up potential followups. if you know his tech game is non existant it may be effective to dthrow him so you can probably make him eat an attack or shield his getup because he is pretty predictable and doesnt tech anything. dont use fthrow unless you are throwin him off the edge with it, up is much better and down against low level players is specifically more effective because its easy to punish their options (since they are unaware of alot of them)
Learn to ledgedrop double laser and use it only sparingly, there are definitely times it could have been applied effectively.
Dair is riskier than Nair, and if you don't have shine combos out of it, then probably throw more into nair approach. if you are coming straight down from above in front of him, if you whiff or on shield you are kinda screwed so sometimes come down wavelanding, or shield or grab depending on what you think he will do, or dair but make sure to hit his shield and L cancel it. OVerall though don't come down from super high up and dair him its risky because eventually he will figure out he can fair you or wavedash back and see what you do. if you dair, you'll probably eat an fsmash or a ftilt or get grabbed. Plus if you are that high you've probably wasted your double jump meaning its risky as hell to get caught in marths juggling game because if he gets you off the edge as falco its over.
If you throw an empty short hop in front of him, grab when you hit the ground, or roll behind him if you are reading it for a mindgame, like if you predict an fsmash because he spams it all day. Or triangle jump behind him and go for uptilt, even on shield its pretty safe.
Also if Marth spams Fsmash, especially low entry level marth players, their timing is EXTREMELY PREDICTABLE. Spotdodging can be an extremely effective tool if you use it ONLY when making reads. Especially with Fsmash he will be left open for a good while which is why you should try to pick up his timing because most low level marths don't try to stagger that at all. it becomes very predicable and you should set yourself up for the punish as best you can
Don't be afraid of marth on the edgeguard (when you have him off the edge LOL not the other way around). worst case scenario you take a light hit off the up B and fail to edgeguard him, but dont sit in shield afraid to punish. Dsmash has lower body invulnerability, rising dair is effective to trade in the air with non sweetspotted up B. You can also ledgedrop and rise into a dair for trade. All these instances you had alot of time to get into position though, try to get used to running into position, wavedash back to grab the ledge. Against marth, if you can time the wavedash back and grab it when they are low, then time the roll so they are forced to land stage while you have invulnerability frames during their Up-B from the roll. Time it right and it will be spaced to where you can hit them with an Fsmash before they can recover to even put up their shield. its alot easier and safer for you than trying to firestall and then drop and jump into bairs etc so try it out and it will probably work wonders for you. If he comes high come out and hit him with a bair, or if he is further out, nair, and if he is low enough and within range, then dair.
Don't get caught charging any smashes, unless timing against a getup or a recovering opponent its probably best for you to concentrate on timing it right and almost never charging your smashes until its read based every time because you will constantly find yourself getting caught. against marth with his incredible grab and range off attacks, he is sure to wait outside your fsmash range and punish it easily after you let it fly.
Its dangerous to roll into people especially marth/roy because the arc of their sword. learn to space with wavedash and bait with it. You can dash in WD back to bait him, even trying to roll past a marth is dangerous if they can wavedash backwards, they create alot of options for themself while staying pretty safe and you can get f*cked.
most importantly though, just learn to read him and SHIELD AT THE RIGHT TIMES. then learn your out of shield options. and your spacing at 1:38 in MARTH SUCKS was great. abuse his tendency to attack and follow it up with lasers, then when he doesnt predict it come in with the air attacks, fsmash was bad.