Stop nairing like that. Seriously.

[Dandy_here]

Stop approaching with nair. Seriously, don't do it. I'll just shield or wavedash back or dash away or something, and you'll probably die because of it. Doing an approaching nair is very easy to punish with a shine. For example, Mango is under PPMD, PPMD throws out a nair, Mango shines and ends up getting the kill. And the worst part about it is that marths tend to do this a ton. They'll come at you with a nair over and over and don't see that this isn't working. It's all they'll do. It's similar to foxes only nairing because mango likes to do a lot of nairs. And to fix the problem, all they need to do is cut down the amount of times they do an approaching nair, and do something safer like a dtilt, or just bait out a move.

 

[AirFair]

I think approaching nair, when a marth will just nair into an opponent's shield or try to cross it up with it, is not a very effective option as well. It's easily avoided and can be taken advantage of. This does NOT however, mean that nair is a bad approach option. Contrarily, it can be a really good way to cut off space and approach an opponent not necessarily going for a hit.

The hitbox of nair, shown above, does not cover directly below marth, and it's obvious when you look at it. Moves like dtilt are much better for grounded approaches, and there's not question about that. So why do marth's use nair a lot, and so unsafely? It's not impossible to use correctly, so there's definitely some misunderstanding here.

Let's look at the Marth/Fox matchup for an example. You are correct when you said that Mang0 could get the shine under PPMD if he threw out the nair. But that's because PPMD(at least in the example) threw out the nair at the wrong situation. Foxes can easily maneuver under nair's hitbox and shine, and here is an example that isn't the one you are describing
https://youtu.be/OC_mcXzGZbE?t=1m40s
Arc did a nair after getting his shield baired. The reason this was the incorrect decision was because not only did the back of the hitbox completely miss Lucky, but he was also able to run under him and regain center stage.

So when should Marth's be nairing in the matchup if it's so hard to hit Fox with it?
There are much better posts and discussions about this same topic by much higher level players, but I'll try and put what they said into my words.

I believe that nair is a move that is best used to cut off options for the opponent, as an extension of your dash dance. The one place that nair finds the most success is in the corner. The reason for this is because when Marth threatens Fox's limited space in the corner with his dashes, most Foxes would want to jump, to the platform and away from Marth's grab/dtilt, and that essentially covers it. When used in other positions, it's used as more of a way to control space, but because of how unfavorable it's hitbox is, I wouldn't really recommend it there, unless you were sure that Fox wouldn't be trying to move in, and thus establishing respect for the move (which imo is better done in the corner) and if you watch top players, you will find that their play will generally agree with that.

Here is an example of good nair usage:
https://youtu.be/K6E6Zp5sYvs?t=2m13s
Note that Armada doesn't get hit by the nair, and when PP moves in, Armada jumps up the the platform, and full hops over into center stage, and PP reacts to him coming down with fair into fsmash, which wins him the game.

To conclude, nair is a good approach option when spaced properly, and when it can threaten a stationary or limited opponent's space.

 

[DieHard]

Personally one thing that I feel is very overlooked with nair (especially in the fox mu) is opting for shffl nair over ac nair
A lot of marths just automatically go for ac nairs becuase "oh the video said this was good!" or some other dumb reason like that.

However I life shffl nair as a way to stuff fox in the corner as it covers lower right in front of marth, (assuming the ff is done later so that the second hit covers a lower area), acting as a way of out ranging fox's nair approach or dash dance.

Can't really find any video examples of this working because no other marth's really use this option, but I encourage others to try it out and test its effectiveness.

 

[FE_Hector]

Instead of talking about the nuances of nairing as an approach, I just want to point out that most of the time Marth's shouldn't really be jumping when looking for an opening, anyways. Armada's said time and time again that the biggest thing that separates PPMD and M2K from every other Marth in existence is that they stay grounded for the VAST majority of their games and don't jump at all except when it's actually a good choice. Instead of jumping for an approach, use this character's honestly insane grounded movement and superior hitboxes to just force yourself into a good position and earn a clean punish from there.

 

[AirFair]

Instead of talking about the nuances of nairing as an approach, I just want to point out that most of the time Marth's shouldn't really be jumping when looking for an opening, anyways. Armada's said time and time again that the biggest thing that separates PPMD and M2K from every other Marth in existence is that they stay grounded for the VAST majority of their games and don't jump at all except when it's actually a good choice. Instead of jumping for an approach, use this character's honestly insane grounded movement and superior hitboxes to just force yourself into a good position and earn a clean punish from there.

I do agree with this. The post that I made above is kind of justification for why nair is used as an approach at all, and why it is used as "a good choice."

 

[ChivalRuse]

Nair approach has a stigma associated with it because scrubby Marth players tend to spam it on auto pilot without much forethought. It actually has an advantage over fair in that the hitbox lasts much longer from start to finish, making it impossible to whiff punish it until after the full move ends. This allows you to force your opponent to change their rhythm, which can be very useful at times.

Undershooting nair can be a useful bait as well. Auto cancelled nair and dash back. For example, : https://www.youtube.com/watch?v=Kb9bVcU7-Y0#t=0m58s

 

[-ACE-]

Shieldstop nair is great when you read an approach, and not as big of a commitment as an approaching nair. Nair in general requires a read but yields considerable reward vs fastfallers.

 

[AirFair]

I like to just fade back if they are going to run towards me so that I can space the second hit well.

Shieldstop nair is great when you read an approach, and not as big of a commitment as an approaching nair. Nair in general requires a read but yields considerable reward vs fastfallers.

Yeah, for the most part, you need to catch them when you know they will jump, which is why it's much stronger in the corner when that's one of the only safe options that they have.