grab marth. a lot.
get grabs by forcing people to shield with needles and aerials
wd out of shield into grabs when marth fsmashes ur shield
shieldgrab if his spacing sucks and he misses l-cancels
and you can combo to grab too, but getting that initial grab is the big thing
once they start worrying about getting grabbed, you wait for stuff like sidestep, roll, jump out of shield, and you punish accordingly
you should always dthrow whenever you get a grab, and depending on how they DI, you do different things
if he DIs behind u (or not at all, which is basically the same) at a low percent (~25%), u can pop him up with a usmash into a fair or tilt into a combo. u can also sh nair
if he DIs behind u at a higher percent, just turn around and sh/lh slap, depending on how high he is in the air. you can still try to usmash, but if they're fast, you'll miss.
if he DIs in front at a low percent (~25%), you can actually chaingrab him until around 30%. when his damage gets higher, then just do dash attack --> sh fair.
now for high percents, like.. 100+%, get your kill.
if they DI behind, sh backwards --> uair or fair. avoid bair, it's weak and won't do ****. if they DI front, then sh fair. uair can also be good, especially if they're not expecting it and they're DIing up for the fair.
sheik has so many options, it's ********.
needles are good.
watch out for his fsmash range.
when youre recovering, or get thrown off the stage, don't use your jump right away. always sweetspot the ledge from below.
crouch cancel his fairs with a dtilt, and combo into a slap.
**** marth when he lands from an aerial. abuse his lag. grab him!
rolling behind him is actually really good. do it when he's rushing at you, then grab or slap the back of his head. whatever.
u can also edgeguard marth. bair/nair off ledge is pretty awesome. bair his face between his fair swipes, and trade off your nair for his upb whenever he's about to come up. needles **** up his momentum as he recovers. cc his upb into a dtilt or dsmash if he tends to miss the sweetspot and goes slightly above.
if you're too aggressive and you don't fully know what you're doing, you'll probably get *****. this match-up may be in sheik's favor, but marth will win if you're not smart about your game.