Marth and Shiek is like a perfect match
marth vs Sheik is a ****ed up match but it is doable. That guy is very right. A lot of up throws into up tilts. If you can get her to lose her jump, you can really keep her up there. The sword dance works well on her too. If you grab most of the game then around 80 % come in on her, she most likly side step and thats when you do your sword dance and she will most likly get hit by the third swing which will send her flying. Try to follow your cancles with a down tilt. The sheik will try grab you after arrials but that down tilt will mess her up everytime
Grabs and juggling; yes. It's also helpful if you can edge-guard well. The key to edge-guarding a shiek, however oxymoronic that may sound, is to keep up the pressure off the platform. Always try to take the edge-hog opportunity, and take it early; trying too late could end in your doom through a well timed slap against the side of the level. If you're not into that, then the idea is to pressure her into using the vanish before she wants to. Those invincibility frames are what makes her so hard to nuke otherwise. If you can force her to miss a sweetspot, you've got it nailed. B-airs out of edge-hog work nicely for that; which is why I advise taking the ledge. Otherwise, you'll just have to rely on your range to deal the damage to Shiek; it's easier to send her out with a well placed f-tilt than an f-smash. Those shieks are always looking for an attack to punish, and Marth's f-smash is the one that makes their mouths water. F-airs are nice, as are u-tilts and d-tilts; d-tilts to push her out of grabbing range. And of course, as always, if your opponent gets predictable, make good use of the wavedash to f-smash. Mindgames > Technical skill =)