more dash dancing. Can use lasers for pressure/percent advantage/kill percents. Beyond that, you just have to have a solid understanding of your punishment openings and options. Ideally, I try and pressure them into dash attacking. Practice Drill shffling into grabs/tilts/shines and practice your out of shine maneuver. Try and push the speed that you play over your opponent (as in, try to process information on a higher level than your opponent.)
Definitely learn your Lcancels inside out as well as general movement lag, such as turning/jumping/landing/wavedashing/rolling/fsmash/ ETC ETC ETC. The more comfortable you are playing at the games max speed (60 frames a second) the more in control you'll feel when playing vs. other people.
You shouldn't look at your problem in a fox vs. marth perspective, since your main problem is your play style against your opponent's.
You likely have little knowledge in this regard which is really the root of your problem. Psychology of SSBM players is not something you can passively learn by watching a video, even if it's two of the best. What you can do is adapt their execution and try to build your own play style that is flexible and patient enough to deal with whatever JOHNNY THUNDER51 is gonna throw at you. Whether its rolling into fsmash over and over or dash dance camping for grabs like a pro.
You might think you're moving extremely quickly but your opponent probably, literally, sees you coming.
drill and shine are great moves but it really comes down to spacing. If I hadn't said it before, jumping in to marth is not exactly smart or safe. However, you can use jumping as a tool to threaten an aerial approach. If you see where I'm going with this then you're in good shape.
Marth has way more range than fox but fox definitely has mobility and good CQC ability. If you don't know how to, learn to crouch cancel the inside of his fair, this is one of the best ways to get inside on marth. Another solid option is scaring them into shield for a decently easy grab or a cross up.
Don't stand around, hesitation will get you fsmashed or dash attacked or worst of all, grabbed. Spacing FOR back air is more threatening than actually throwing the back air out. uhmm
That's all I got off the top of my head.
Try to pressure with spacing in a way that offers marth an out or an opening, this way he'll be more likely to actually commit to something, whether it's a full jump or a dash attack/fsmash/ etc etc etc.
For the love of all things technical, don't **** around on the edge, off the stage, near the edge/etc etc etc. because solid marth players will gimp you and before you know it, you're out of stocks.
edge guarding:
This is tough. Especially if the marth keeps his double jump. If he doesn't have his double jump the light shield edge hog (if you don't know this yet then you're on your own) is usually a stock. Alternatively, there is a small window (1 frame) where your feet will beat marth's UP B with ivincibility.
A great opening for shine spikes happen marth's try to stall with forward b or double jump into an aerial (you have to be threatening to take the edge really well to get this reaction). On a similar note, You can pressure Marth players into using their UP B EARLY (recovering on to the stage vs. onto the edge), again, by threatening to edge hog.
Some marth's like to stall with edge hanging and this tactic is hard to counter if you're not careful. If you don't feel up to it and you can't develop a way around it, ignore the edge. If they aren't going to get off (they probably have a stock lead) You're going to have to be very creative to turn the situation into your favor without losing stock/taking tons of percent. ---
I brought this up yesterday and I'm sure you've all fallen pray to this **** before but running off in an attempt to shine spike then double jumping in an attempt to get back to the stage only to be sniped by Marth's UP AIR. I recommend wavelanding into a roll or something!!
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ok, Definitely lost my train of thought here.
Honestly, I think that should be plenty of info.
tl;dr: you're playing vs. another person. Keep that in mind and try to be creative in your attempts at removing him from the platform 4 times. Drill, shine, and grab are bread and butter vs. marth.