Falcon shouldn't get a grab without stopping you from moving first. Keep moving and shooting.
Marth and Sheik shouldn't get you still unless they PS you first or trick you another way. Sheik SH'ing is something you should be prepared for.
Don't shield at neutral at all unless it's a bait.
vs Falcon on plats, shoot lasers low sometimes high, FH/DJ trick and falling laser or rising/falling Bair. sometimes shine if you see him moving and get a good read. trick him into thinking youre jumping then utilt/bair him. standard offense vs defense stuff. don't be afraid to platform vs him either.
if marth spaces well on you then just act after his swing unless he's smart and feints into grab sometimes. then just try not to hold shield too much.
falcon spacing you either wait for gentleman or gtfo asap too.
avoid shielding if at all possible and instead try to counter with utilt or bair or laser or nair out or fh away if they get close. shield sucks lol.
don't follow up on sheik, just get away. rolling away or WD'ing oos away is fine esp if you do it early so they can't chase and you have time to set up a laser. just don't overdo it.
vs fox, set up lasers to hit his landing spots and otherwise bair/utilt to beat his approaches so he has to respect your moves. when the pace is slowed you get to chase him then usually.