If Falco messes up the height of his laserhops, you can also run under them.
Marth is just a matter of spacing and sometimes CCing some attacks. Punish his lag w/ dash attack, tilts, and grabs.
Jiggs is kinda weird. As Bing said, look at KK's Winners set against HBox at Apex. You might also wanna check out his Losers set, but he lost that one, so I dunno... Basically, I like to zone with bair and fair against her. Grabs always combo into uair or fair. Use needles, too. Needles give a good amount of time for hit-confirmation and they often combo into grab, ftilt, or dash attack. Be wary of this: Jiggs can trade with or beat out all of Sheiks aerials. Also, Jiggs can CC almost all of Sheik's grounded moves and can punish most of them with rest.
As for Fox, do NOT just throw out moves hoping they'll hit. That's what DDing Foxes want you to do. Their DDing lets them maintain good spacing with pinpoint precision while retaining the option to punish you once they get the chance. Frantic dash attacks and dsmashes will only get you punished. Throw out needles and try to hit confirm them into stuff. You can also try to poke in with an ftilt and dtilt now and then, but a lot of the time, you're gonna be dashing around as well. Also make sure to utilize your AC fairs. If you observe that they like to shield when you aerial at them, you can try AC fair>grab. If the Fox is consistent with his shines, then don't try to shield-grab him after his aerials. In fact, you probably shouldn't be shielding much around him at all. Your protection is your mobility. Stay on the move. If you shield their shine, nair or WD oos. Laser spamming shouldn't be too much of an issue, and neither should trading.....unless it's usmash. @_@
Be wary about using dsmash much. Puff can CC it and rest Shiek out of it. It's happened to me...