Marth Matchup
Sorry to double post but I'm an experienced Falco v marth player at the high level and have done at least a years worth of dedicated Marth theorycrafting since he became the character I started running into trouble with in tournaments. The matchup forces falco to use tools and reserve himself in a way that doesn't really come out in other matchups.
I'd like to point out that orvs information above has principles to live by. Marth is falco's hardest counter by far and one of his worst matchups because he loses all horizontal advantage against Marth at ranges where lasering is dangerous and can't abuse his vertical mobility or priority as freely as he can against other characters. Moreover he is always at a first game disadvantage against the stages Marth gets to strike to, if you can convince the Marth to gentleman to Battlefield you should until they stop falling for it, thank Hax because he has everyone convinced that's the most neutral stage for
every matchup and people forget that there are catch-22s you can get forced into if someone strikes it.
The run up shield advice is not advice I would give because it restricts movement options you badly need and takes away crouch cancel, and it's a visual cue to the Marth that he should go for a grab right there. As always you can punish with spotdodge shine but that is only if the Marth spaces badly, otherwise you have to jab and dash away, which means you just lost stage.
This seems grim because on paper marth does win the matchup, but you just have to be better than your opponent inside the game. On the brightside, there are two areas within the matchup you can abuse, being in the center with laser pressure or being right inside marth having an RPS battle. You have access to shine and his OOS options are slower than the average top tier, and this is the only point in the matchup where you can do to him what you're used to doing to other characters and feel comfortable. When you get inside make the most of it, and when you're outside you need to be fast enough to react to every move Marth makes and punish every jump he does (jumping is slightly more disadvantageous than being grounded in this matchup, for him). Discipline yourself and reign back, do not just laser nair. Space outside him
with lasers and a back-up plan for WD OOS (marth likes to invade your space quickly and grab if your lasers are predictable) and test different timings, bait and punish, force things from him. You are not going to beat a competent Marth when you are the one forcing moves into him from neutral.
If you want to practice tech skill for this matchup, focus on dashing as soon as possible out of moves. Dash away after laser, dash away after jab (great in this matchup), turning your dashes into dash wavedashes, reactionary spot dodges upon landing, delayed lasers out of full hop to keep your landings somewhat safe (don't rely on it when you fight Marths at tai's level and +, you'll find out why lol) because it will keep out a jc grab attempt on the spot you land. Practice ledgedashes as well because you will need the invincibility, LHDL is a habit you have to break against marth. this is enough to get you competing with top tier marths if your tech and punishes are up to 20xx standards, just live by the principles of the matchup because Marth will murder you off an opening.
I kind of poured out a lot of the secret tricks of the trade here because I hate marth and the less of him that exists in the tournament ecosystem the easier my life is haha