You don't have to react to the Phantasm. You just have to react to their height. If they are above ledge height, you just wait. If they Phantasm, well then lol they'll get destroyed. If they up-B, then lol they'll get destroyed. If they drop back to ledge height, you dtilt. Even if they shorten perfectly they get hit, and if they up-B at or below ledge height, you can easily just run off-stage and gimp them because of dtilt's IASA frames.
As far as neutral B, like I said, I'm not sure it even has a longer lingering hitbox than any of Marth's other moves, and even if it does, it doesn't have the range of dtilt required to beat shortens.
don't actually know much about what the best option is for punishing phantasms but i wanted to say like 3 things.
first of all, what is the start up time of phantasm? everyone claims you can react to phantasm, but my testing for 30min on really close distance phantasms did not agree. i was killing the falco pretty much every time but i was anticipating based on tests. this is based on height like bones said..
though at full distance you can react easily.
however,i should point out that it is technically possible to time phantasm in between the first and 2nd jabs if you get the marth to jab early (when you are at head height or something) and then go early..but in reality if you jab twice the majority of the time you get them.
so with this glaring hole in marth's edgeguard game will falcos attempt to break the interjab window as i so thoroughly tested was possible?
no, they will try to shorten instead because your chances of living are a million times better. alas, another pointless technical piece of data that has no relevance on the actual game....at least i know i've contributed to the marth compendium of useless knowledge
on a more useful note:
anyway, i punish shortened phantasms with double jab, but correct me if i'm wrong, but if i do not wavedash to the ledge first before double jabbing, should they reach the ledge if they fastfall? i'm pretty sure most of the opponents i play with don't fastfall after the shorten
and the same thing with dtilt..of course dtilt will win if you get to the ledge, but honestly there are plenty of situations where you don't have the time to just wavedash to the ledge and they will reactively phantasm through your wavedash if you try since it's technically lag...
and i question how many of you actually wavedash to the ledge when you see the falco jump back and phantasm..or do none of you fear eating the dair/wavedash on shine from falco?
it may cause more problems than it solves if you attempt to edgeguard like leffen suggested...but i agree those will cover the phantasm flawlessly without miss.
are you really going to go all the way to the ledge and take the risk of them getting a sweetspot and retaking control..and if any of you have played pp, you should know that falco getting control = half a stock (unless you're on a big stage and then it's only 1/3 woot!!)
on a side note fair jab is better than double jab, the reason people don't go for it is they fear messing up and eating a phantasm while in the air and then dying off their own failed edgeguard attempt ( i don't try it either..but strictly on frame data and hitboxes it should be better)
also in response to niko, based on the frame data it looks like shieldbreaker hangs 6 frames instead of 4 like the other moves....so it really should only be covering the cancel if it takes less than 2-4 frames (since the first two the hitboxes aren't really in the right positions) of fall time...so i get the feeling it wouldn't work against high phantasm cancels but it might work against low ones but i have never tried this so i will play with it some this week hopefully.
but yeah i can't picture this actually helping most of the time if the phantasm cancel is done more than a centimeter above stage height.
but testing will show..i will test it at some point..thogh what would really be nice is if people would test it with AR...that marth's edgeguarding hitboxes is like the best video ever made for marth....i like find myself referencing it every two days..
if we had one demonstrating edgeguarding hitboxes vs shortened phantasm..life would be perfect...if only i had an AR setup..maybe when i go to nova i will talk with sypher