Well marth is hard, but I got a lot better just thinking about what options I have. Like crashman mentioned in my thread to dash attack his arials, it works. You can also crouchcancel his forward airs.
Now put these together. Crouch cancel a forward air, and they will most likely Nair you next time. But next time you dash attack them, and then you can possibly juggle them with this. Try popping them up in the air because you can outspace them much easier from below.
Sheik you can't really get out of combos, they are just automatic. From talking to Wes, HugS and Watty about this match, some reoccuring tips were use your shield a lot, only jump to attack after they jump.
Falcon can really only hit you with 2 hit combos most of the time, just sex kick out of them, and only use sex kick to counter their arials, but feel free to use other attacks vs their shield. Also don't Dsmash their shield if possible. Falcon you will want to pop up in the air, tech chase him with charged shots and such(Wes told me this stuff). His roll, I have been told, is slow, so this shouldn't be a difficult thing.
Jiggs is the same as falcon shield wise, she can punish most of your attacks that hit her shield. Try more arials and missiles in this matchup.