When you play against marth, make sure you capitalize on every opening the fullest, taking a stock or at least doing massive damage, because you can be **** sure he'll do the same. If you miss your edge-guards, work on it until you don't. If you ever miss waveshine->grab, work on until you don't.
Every grab you get should be followed by massive ****. Up-air is good at low percents when bair won't give enough hitstun to follow up, and at high percents to kill. Bair is good at mid percents, to extend combos for max damage.
If your opponent can SDI out of up-throw->up-air with any consistency, then killing marth becomes much harder. To get around this, learn to space your up-airs so that only the second hit lands. Roughly, the way to do this is to double jump and up-air at the same time, positioned so that you barely reach them.
As I said before, edge-gaurding marth is something you have to master. The basic idea is to get on the edge and keep renewing your invincibility. The most common mistake here is to get off of the edge too soon. This is understandable, given that if you get hit by his up-b, you're pretty much dead. You j have to remember that when you have invincible frames, you are, shockingly enough, invincible. Just wait until they are within the range of a low edgehopped bair, then do one. As an extra precaution, in case your frames run out to soon and you get hit, you can press L as you bair. That way, you will tech, walljump (I'm assuming you jumped with up like me), and can side-b to the edge again. Either way, get back on the edge, rinse and repeat. If marth tries to go low, then just drop and shine him. Finally, if he stalls right below you by doing side-b double jump fair, you should be able to drop and safely shine him in his lag, assuming that you have pretty good reflexes.
To get openings on marth, there are basically two different approaches. One is run away and to shoot lots of lasers to force him to approach. This works as long as you keep your goal in mind: make him approach, and punish it. This means, when he does approach you, don't run away and shoot more. If you do this, you'll just get pushed towards the edge, where you'll get molested until you're dead. I generally shoot lasers only from the center of the stage, when my opponent is near the edge.
The other method of approaching marth is to get up in his face. Dash dance around him, close enough to put pressure on him, but out of his sword range. If he goes into the air, get under him and ****. Running sheild->stuff and dash-attack work well here. If you dash-attack, make sure to crouch cancel in case you trade. If marth tries to dash dance camp or run away, overshoot grab. If you get really close to him, spotdodge->shine->grab is amazing. The idea here is basically to out-maneuver him and force him to make the first move.
You have some right ideas but your edgeguarding advice is very dangerous.
VS Marth you want to use dash dancing to space and have him miss an aerial, a grab or a dash attack, dtilt is only really punishable if you were already attacking over it, so its not reliable to try and punish, utilt is also difficult to punish because they shouldn't be throwing it out at times you would be able to. Fsmash is punishable if your quick with a wavedash OoS.
So how do we get Marth to attack us and miss? well we laser and we run around, I like to use platforms for this too, but you have to be really patient, its a dangerous place to be, above marth, but I find I get grabbed less up there so its sort of a trade off.
If you can get a shine off, a nair, or a drill, they should all lead into grabs in some way or another, dair you can shine wavedash grab uair, or you can just straight grab if you didn't connect vs a shield. With nair at crouch cancel percents I shine just to be safe, but those often lead into grabs, I think its guaranteed around 40%. Once your opponent is used to being grabbed out of nairs you can slip more nairs in which they will usually jump into trying to avoid the grab.
Edgeguarding Marth is tricky, you have a few options, if they are close enough and have a double jump and are able to sweetspot or make significant distance onto the stage I would be very wary, you MAY be able to land a risky shine but you could also die, also if they attack in an unexpected way you are going to be in hitstun at the edge, which is awful, especially vs Marth. If they are so far they have to sweetspot lightshield edgehog is guaranteed, also if they are over 100%, lightshield edgehog should net you a kill even if they land on the stage, just ledge hop an uair into them.
After about 90% I like to sneak bairs and nairs in after my uthrows to get them off the ledge and get some medium percent kills, after a couple of these people start to DI up which is death 20% early once I mix in uairs again.
Other than that its all pretty much up to you to predict timing of their upb, I like to stand and dsmash if I think they are gonna up b early, if I expect a sweetspot I try and force myself not to move, I know its reflex to roll or get off as soon as they are in sweetspot range, unfortunately Marth has 2 tools to extend his sweetspot duration, a double jump and a forward b, if you predict either of these you can often shine spike them during it, if they already used their floaty forward b or double jump then you should be able to get the edgehog by being patient, although if you are at a very high percent its risky cause you could be spiked into the ledge.
Make sure to mix up your recoveries as well, fire fox above the stage at angles that will let you go towards your opponent, sweetspot or above for a platform, make your opponent guess. Don't do it so close they can jump out an fair, neutral b or spike you.
whats fox's best way to punish spot dodges? i was practicing vs aarosmashguy and he kept spot dodging my missed grabs...but then when i started to wait for his spot dodge, his shine would hit me before i could punish it. :S
The timing will vary based on your personal reflexes, I hit grab almost right after I see the spot dodge and they usually end right when my hitbox is out, I run at them in shield, wait for the spot dodge, the second it goes off I have a VERY minor pause and hit A to grab them. This is the timing I use, you need to find one that works for you.
Also Upsmash, charge for half a second and release and it will usually land, the problem I have is that the people I play spot dodge less the high percent they get, so often if I do have an opportunity to punish a spot dodge its at a lower percent where Upsmash won't typically link into anything.