Ah,now we're beginning to delve into usage of attacks, dealing with specific options and whatnot:
Aside from just poking, run up dtilt is best vs the back end of their dash, as was just stated. If you're not confident in your timing for this, or any other type of precise approach (like run up grab), it's probably better not to dash there in the first place, but during initial dash, shield stop is harder to punish if it whiffs.
From the shield stop, fairs or nairs in place or retreating can beat most attempts to hit your dash forward, and are harder, but not impossible to punish than jumping in outright. Approaching with a shield stop grab spaces it so you don't fly into the opponent if you miss. Again, these aren't unpunishable, but they make the opponent's job harder, and do a degree, the fair can be used for a gradual approach somewhat safely, but don't swing if it won't hit.
Grounded options can be great to deal with the opponent's jump, but be sure you have enough room for whatever you choose to actually come out, and try not to whiff. If they're jumping towards you, and not overshooting, it's usually easier to just dd grab, but vs in place aerial walls or retreating aerials, it's basically asking to let Marth throw out a move with more start up but more range, because they're not threatening to interrupt the start up. For example, if Sheik is doing AC fairs, or Falcon is nairing in place, if you're not in range, or won't be immediately, then rather than shield, if you aren't already, or dash away, you can just... Jab, uptilt, ftilt, or even fsmash. Jab's obviously not very good on hit at lower percents, but at least they can't CC from the air. Fsmash is the laggiest, and so worst to miss with, but if you're confident in spacing, you only have to worry about an air dodge (and this isn't even always the case). So the intermediate option is to throw out a tilt, but of course, you can also go for tipper aerials and drift in response to their launch. At lower percents try to drift so they can't mash out an attack and hit you, at higher, try to follow them and press advantage.
This doesn't just apply to aerials, but can be seen as better vs them due to how motion can be influenced as hitboxes are coming out and present from the air, but any move with a tight window to come in and punish the hitbox for whiffing can be beaten by just using your own hitbox that beats it concurrently. You need to be doing a stronger move to beat grounded moved this way, however, and their options are more variable and harder to respond to from the ground. End lag is basically set, however, so for these, it's best to get that exact spacing you won't get hit, then come in after, usually.
The best way to deal with shield is to simply not be in range for any OoS option to threaten you (or at least not in such a way you can't react to and punish), then be as close as possible to best respond to whatever they do. Of course, the longer you do nothing the more time they have to realize what the safest escape option is if they know your plan, and if you're focusing on being in that perfect safe position, you're making it harder on yourself to respond to what they actually do before you get there.
So, to more proactively deal with shields, obvious options are to grab, and do a fully spaced dtilt. Of course, the grab can be dodged, and the dtilt, even if an unstaled tipper hitting on the 9th frame, still has extremely strict types of punishes that can be done vs a buffered roll back from say, Falco even with perfect reactions or foresight of what the opponent will do (you can't grab Falco for this, but a WD tipper fsmash can hit before he could bring his shield back up). Shield DI in, too, makes most "safe" attacks punishable in some way. To try to mix up this tactic, nairing through the opponent suddenly brings a move usually thought of as unsafe and bad vs shields to be one of the better tactics. Of course, rather than wait and watch, or play this mini-game that looks like something of a mix between RPS and a game of chicken, you can simply act first.
This is a place for borderline, if not outright preemptive grabs to come into play, only spaced. On approach, while stopping when you see the shield and waiting is best, if you shield stop grab, or jump ready to fair or nair, you cover all sorts of options, or at the least do a better job not getting punished. You can fair any attempt to move forward, and nair will also cover most forms pretty well, if you drift appropriately, and both punish spotdodge. If you grab and they spotdodge, the options they have that can punish you for missing tend to be very limited, and DIable or CCable. Shield stopping outside of your range to grab them so you can punish them for reaching is also great. Grab their shield grab, do an aerial on a spotdodge or attempt to move forward, wavedash after a roll away. That won't allow you to punish a wavedash back, however. For that, you basically need to not have gone into shield.
So I've said a lot about uses and counters to aerial walling from both sides, and one of the options I've alluded to can even beat ground walling if executed well enough. Shielding is best beaten by waiting I said, while I was talking a lot about the dangers of spotdodge, and how positioning one's self to throw out attacks is good, grab is still the next best thing. So, run up-shield, + Marth's amazing grab range, logically helps you deal with a number of attempts to wall you out, but also gives you room to deal with other options including the opponent shielding.
Dashing towards the opponent in neutral generally leads to them trying to interrupt your advance by throwing out a move, or going further back to try to catch you at the end of it. So of course, run up shield goes against this mentality of urgency and danger, if they try to interrupt you, they may have just fallen into a trap. If they dash back, well, you might not get a direct punish, but you can still try to track their movement. You can wavedash out of shield at any time. More importantly, however, them dashing back means you get stage position.
Now as it's been presented thus far, shielding appears to be a powerful tool, and it is, though limitations have been mentioned when discussing frame commitment to movement. Still, if executed correctly, it beats all sorts of forms of attempts to wall, if done out of a run, the opponent moves forward in shield. Combine this with the fact is comes out frame 1, and they can just keep moving towards you if you dash back, it becomes apparent that just waiting it out might not be the thing you want to always do vs approaching shield. So what can you do if even a dtilt isn't safe, and other ground moves become extraordinarily unsafe, and exceedingly diffciult to space? There's so many variables that can affect Marth's spacing sensitive moves and make them not work out how you might want on hitting the opponent, and it's even worse vs their shield, so perhaps you should go for an option where spacing is actually irrelevant as long as it actually hits, explicitly beats shield, and minimizes opportunities for any type of DI?
Only one move even comes close to meeting any of that description above: grab. Yes, just timing a grab works fine, but pivot grab is so much more powerful. In the same way that dashing towards the opponent creates some kind of threat they tend to respond to pre-emptively, or set themselves up for later steps, dashing back makes the position where the opponent can hit you from even harder to determine. They continue moving forward, possibly only expecting a single dash back, and spacing for you to come back at a certain point to try to hit you again, or they're ready to spotdodge after the shield comes up and they see you dash back. That's fine, you don't have to let them do any of that. You dash back. Once they're in range, you immediately turn around and pivot grab. If you miss, it's almost certainly because they didn't get close enough. Not that big a deal, but still, you shouldn't miss. If they try to continue chasing you, they have no hope to react, and even if you're doing it at pre-determined spacings for them to stop at, you're more likely to come out on top.
Not further elaborating on things like dashing forward and dashing past because this thing is already long enough, lol
