I suppose I'll state the obvious 2 scenarious:
a) You mind game a whiffed f-smash.
b) You shield the f-smash.
for the first scenario (mind-gamed whiff), I like to run in and get a grab, because for me, everything about this match is about getting the d-throw, and getting that aerial in to get Marth over the edge. For actual combos, after I d-throw I:
1) 0-15% - do another d-throw, then shffl any aerial. If you d-air, punish the tech/get-up recovery with whatever you please. If you u-air, follow with another u-air if you can. This usually will get him out over the edge and let you edge-guard for an early kill.
2) let's say you fail to chase, and he's at a good 30%-50%: If you're on the inside of the stage (i.e: marth is closer to the edge than you) Sometimes I do a wizards foot, but do it as soon as you see that whiffed f-smash. Any slower, and he's likely able to shield and punish. I usually stick with a shffl'd f-air because it's got a nice long reach, but if you're fast enough, you can multi-air a d-air into a f-air/b-air for a good 40% damage and that'll get him over the edge.
3)Marth > 65% damage: I'd stick with the f-air here. You can try the wizards foot here as well, but again, make sure you do it quickly! If you're not facing him and you definitely feel a whiffed f-smash coming out, do a side-b to u-air (I say u-air here because the f-air may not reach the height Marth has been hit to, but if you can hit with the f-air, do it!).
As for scenario 2, where you shielded the f-smash:
1) If you're facing him, shield grab unless you're too far out (i.e tipped). If you are too far out when he hits your shield, you're better off waiting for a whiff, or for when you're close enough to shield grab. After the shield grab, just d-throw (I love that d-throw) to any aerial you wish: I usually just stick with b-air and f-air, because d-air has the most chance of being punished, and u-air may not hit him as far as you would like.
2) if you're NOT facing Marth, and relatively close, do a b-air right out of the shield, and that'll get him off you. If he manages to shield it, then he'll probably try to shield grab you, because you've b-air'd, right? Well, that's why you don't shffl it, and instead just multi-air the b-air. (If you didn't know you can DJ out of a short hopped b-air.) If you use this too much, he'll wisen up and just shield through your second b-air. And then you die. Not cool for Ganon.
Mix it up a bit and go with gut instincts.
I hope this helps, good luck!
lol