Although not a walk in the park, I really do like the marth matchup. I am in a PR region where the PR 1 is a marth (my goal is to TAKE HIM OUT!). I've faced both kadaj's marth (once) and lost narrowly and HRNUT's marth (lost on pools barely but I won one in a friendly afterwards before brackets were up).
I'd have to say the most fast-falled ANYTHING... never really works in most situations of approaching for me against marth. Marth just dolphins both d-air and ff f-air if you are within kill range or takes it with shield/spotdodge and sword dances which almost always comes out before much anything else. Most of the time, marth is f-airing within that range so rarely will you find yourself above him and be able to fast fall through a zoned marth f-air.
Fast falled stuff work more when juggling, falling through platforms, and when you are defending and counterpunching--not as much when pika is approaching.
On the marth matchup, reading is everything (but then again... it applies to everyone else... lol 
). Marth's can approach with:
1) walk(run)>shield>(sword dance); (all purpose [hit/kill/shieldcripple] move)
2) walk(run)>shield>defensive rising SH f-air (tip lands on victim sheild and marth does not advance further);
3) walk(run)>shield>aggressive SH-f-air (most of the time its a mis-spaced f-air where you rush inwards while shielding more than marth expected... that or you are blocking/dodging/avoiding his defensive f-air real consistantly; this less conservative variant of f-air also arises when marth senses that pika is a spamming jolter and wants to swat both pika and t-jolt in one fell swoop)
4) Walk(run)>pivot/go other way>mindgamed aerial/reverse swordsdance (used against an aggressive shieldgrabbing opponent)
5) Walk(run)>mindgamed grab (used against rushing shielders)
Of course there are infinitely many more options, but its a good start for beginners to wrap their head around, since the other marth options are going to result from mostly mindgame anyways (which you just have to read/expect or get owned).
I would try to jolt until marth is at least 1/3 FD distance away. Once marth passes that imaginary point for me, i'd rush in with shield or jump in place. I'd jump in place if i expect a mindgamed grab/swordance/plainshield from marth and try to punish with another spaced tjolt (on ground in front of marth so he can't powershield as well), fast-fall airdodge behind the marth>pivot(d-smash/u-tilt)>grab(u-air from u-tilt), or fast-fall back to ground retreating back a little and pulling out shield as fast as possible. I'd rush in and pull up my shield if i expected anything else other than a grab from marth.
Upon taking a hit by shield, i'd try to do the following based on what i was hit by:
1) Swordsdance: I'd shield the rest with my shield and punish after (if they do all 4 without hesitation). Against the 4th hit down version, I can take it if i have full shield and marth doesn't hesitate). If he hesitates before the 4th, then I'll jump out of shield>rising n-air or grab or something. I keep a sharp lookout for any slight pauses in swords dance and n-air/grab(if i can) out of shield on the instant of hesitation. From my experience in reading, if the marth is a 'no-big-namer' as in not an expert marth, it's a pause after the 2nd hit to look closely for (since most nubs mess up here when they are nervous). If the marth is GOOD, then watch closely more for a pause after the 3rd hit (unless the marth is going for a freak-random shieldbreaker followup which most of the time follows after hit 2).
2) Defensive f-air: Expect an ff airdodge back to ground or another f-air (stationary or retreating back/fowards slightly) which most of the time is to cripple shield further. There are 3 types of marths (of course they can just change up their timing, but for most marths, they have set tendencies for certain timings--at least for the first few fights.... sort of like the advantage of knowing someone is naturally left-footed in soccer... they could use the right foot, but the odds are... left foot mostly for the shots that count):
2a) Marth 1: After defensive f-air, almost always immediately f-airs afterwards. Slight opening: tends to DI towards where marth 'thinks' you'll be--if you move from that position, you wont get hurt and can retreat safely after shielding or spotdodging. If he advances at that point, punish him out of shield (u-smash for example). Retreat(roll back)/continue_shielding if he does not advance at that point. Most of the time, if you have enough composure to shield through all of it, it's either a toss-up with retreating after that point or rushing in with shield/air if you think marth is not going to buffer anything upon landing (with enough shield to take some swordsdance or aerial space to avoid more f-airs). Rolling behind Marth is possible and can bait a reverse swordsdance which you can counter using the examples outlined for swordsdance (which is what i would probably do if the marth was PURELY just doing defensive retreating/stationary f-airs.
2b) Marth 2: After defensive f-air, almost always waits a split second and DI's accordingly waiting for the victim to do something (at which f-air comes out) or when marth is about to land to use f-air(hitbox barely comes out, still cripples more shield/hits victim, BUT MAINLY to autocancel his landing faster for more buffered f-airs/swordsdance/smash). Slight opening: take the first defensive f-air with shield and do out of shield rising/advancing f-air/aerial immediately. Fast-fall after the last hit connects, rush his landing/juggle/whatever.
2c) Marth 3: Perfect mix of both (like only top 5% of all marths anyway). Up to you and your skills. Conservatively, react like (2a) where you defend/dodge/hopeforluck more which should keep you safe from (2b) also if you succeed. If you are daring (can read better), react like (2b) for a 50% chance for a good juggle or 50% risk of taking 1 tipped f-air otherwise....
disclaimer*** these are general trends... of course marths with change it up if they are constantly getting owned, but the change up most likely (from my experience) happens later in the match (if ever) and it changes their 'mood'--so instead of (2a) react like (2b) for the next couple/few situations and it should be fine. If you face a perfect (2c) as in ITS NOT A MOOD/ STREAK thing... then (tough luck...) get better at reading or flip a coin between games... lol, and choose how to counter beforehand. ^_^. To make it easier in your mind, try to force the "aggressive f-air" situation.
3) Aggressive f-air (continues to DI towards/over you after the first f-air lands): I luv these the most. You see these all the time from scrubier marths. PICK THESE APART! DESTROY THEM! You can ANYTHING out of shield and it'll hit them before a second f-air can come out. If they do even ONE of these at killable damage, learn how to SPOT THESE AGGRESSIVE F-AIRS, REACT, AND CLAIM YOUR EASY KILL.
4) N-air: If the first hitbox hits my shield, then i spotdodge the second hitbox>dashgrab/punish or just jump out of shield and u-air/n-air if he's within range.
5) Ground move: I full hop>t-jolt and either give him residual damage (which is always good in this matchup) or slow him down from following up or crippling a little bit of his shield.
6) Grab....: i guessed i messed up reading marth.... lol
So there is my 'general' strategy in approaching most marths. I never approach them if they are standstill. ONLY when they start running or something (that way, if you rush in and shield when they dont expect it, they may accidently over extend their f-air approach [aggressive f-air ^_^] and get hurt).
Hope that helps
EDIT: a spring Grand Finals between me and a PR marth (not the #1 PR but still up there)
http://www.youtube.com/watch?v=BFG910thTLw 1 of 5, Set 1 of 2
http://www.youtube.com/watch?v=POBsATO8i8U 2 of 5, Set 1 of 2
http://www.youtube.com/watch?v=2U9JdM9g9Oo 3 of 5, Set 1 of 2 ~ LDPK's loss (close ^_^)
http://www.youtube.com/watch?v=jBhcJfJPhEk 4 of 5, Set 1 of 2 ~ LDPK's Set
http://www.youtube.com/watch?v=Dl9Z1fAsvQY 1 of 5, set 2 of 2
Theres the winning vid after that set2of2 marth match as a video respose on youtube if you want to watch it--but he switched to wario finally after deciding marth was too hard against pika at the moment.
He wasn't the BEST of marths, but somewhat decent (and unclucky) and it illustrates a lil of what i mean.
![[FBC] ESAM](/data/avatars/l/92/92614.jpg?1365783210){kind=avatar}
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