http://www.youtube.com/watch?v=l2YXFiqOkWM#t=2m28s
My vs Marth game is very weak currently. I've been playing mostly Fox the last month or so, and this is the main match up that I'm struggling with.
My DD game is weak, so I'm finding it difficult to get a solid dair approach in. The Marth players in my state are all solid players, so nair is decidedly weak against them so I'm trying to stick to dair/spaced bair approaches.
What can I be doing differently?
A few things unknown and I saw:
-Down throw
-2:41: Full hop nair when Marth is obviously DDing and waiting for you. Its not that bad if he's airborne (i.e. coming up after you on platforms)
-4:15, why?
-4:26, WHY?! Try overshooting grab, SH nairing and starting some kind of pressure, or just running up to his shield and shine JC -> aerial / grab
-4:59, Uthrow uair combos at this percent. I'm not sure why you keep dthrowing.
-5:24, were you trying to CC shine him? Marth is going to space all the time. Don't do it unless you're SURE it will work.
If you say so, if the Marth won't approach or is just dash dancing constantly, you can just camp them with lasers.
I really hate it when people say you need to laser Marth. Lasering and counter-camping is not a definitive answer in the match-up. Out of the characters Fox is 'supposed' to camp, he arguably has the fastest ground speed and the most range. His ability to chase you down using that range, and safely space large disjointed hitboxes, is unparalleled. Not to mention that once he corners you, it usually spells your doom. His dash attack range is monstrous, and has the potential to start a very, very gay series of events involving knockdowns, grabs, and probably fsmash. He can hit dash attack from mid-range by just dashing once and performing it, since it propels him so far forward. If it hits, it ruins your DD spacing (you'll probably be mid-range) AND starts a combo. You could try blocking it, but It's also safe if spaced. Against Marth, if you get single-minded and decide to camp, you usually end up simply cornering yourself.
Most of the Foxes that eviscerate Marth do so because they get lots of grabs, corner Marth, or edge-guard efficiently. I don't think playing defensively facilitates any of this. As Fox you should rarely be making the first move, but running back and lasering is not how you should go about it. Dash dancing out of his range / around his moves is your best bet.