Sheik — if its somebody who doesnt main sheik, COUNTERPICKING sheik against you, transform and chaingrab them to death. get good at sheik vs sheik. If it's somebody who is actually good at sheik dittos or mains sheik and knows what to expect, then I stay zelda. I lost 1-2 against Drephen in Zelda vs Sheik, I think I could have had it but my biggest mistake was when I got a grab at the ledge, I would do a dthrow hoping for bad DI for a free kick. against high level sheik, just backthrow at the ledge every time. If sheik has to upB to the stage (cause you are grabbing the ledge), you can get another backthrow. you can kick her during her landing lag from her upB to get a kill if she's at high%. beyond that, I really don't have much advice against sheik
Marth — take him to dreamland. marth vs zelda is like marth vs samus, marth needs to keep his perfect spacing the entire match. Marth, if played perfectly, beats Zelda. so you have to force mistakes and be patient. marth has no good aerial kill move so if you can recover to the top platform, do it. lightshield after you teleport, and DI his throws down or towards the ledge. unless he hits a fsmash tipper to get an early kill, you should be living to like 150% every stock. if the marth is good, he will use a lot of grabs. The very best place you can be in this matchup, his behind him while he is shielding. from here he has awful options and you can react or follow what he does and punish it. if he is desperate for kills, he will do a lot of utilts. be aware of his kill options and avoid them. you can gimp marth at like 20% if he doesn't sweetspot, by doing CC dsmash. I'll play with CC dtilt as well and see if I can make anything out of it. I would imagine cc dtilt to dsmash could hit low and be even more effective, but I haven't tried it.
Fox — move fast, get good at perfect WDing so you can maneuver fast enough to get away from bairs and space yourself better against things. you must understand his shieldpressure game completely. If you do, you will realize when the best times to bair or fair are, out of shield. If you get to high% he might get desperate and start doing dash -> upsmash. this is a good thing, shield it and punish with a kick. If he is recovering and is clearly going to pass above the ledge, stand a bit away from the ledge and dsmash him (the hitbox goes pretty high up). Good options from the ledge are ledgehop nair, ledgehop fair, waveland to shield. if you get a grab, chainthrow if there's no platform above you, upthrow to kick once he hits 60% or higher. low% upthrow to nair is what I do when there is a platform above me.. I'm not sure what good options there are when there's a platform there. if he grabs you, and you aren't near the ledge, he will do either upthrow or downthrow. DI both of these throws behind him, tech the dthrow and double jump out of the uthrow. never let fox get an easy kill on you