How to remedy the Ganondorf vs Rosalina MU into a 50:50 for both (no Customs):
-While Rosalina has the Luma Shot ready, be ready to read it with an up smash or a side smash, and if you read it well, you could get a little charge off of it and knock the Luma into a combo hit with F-Air or U-Air. Down smash will not work as its lead-up frontal attack will not knock Luma back to the second hit, and N-air would end up leading into a Rosalina punish out fo the three usable aerials. B-Air and D-Air are not recommended as it would take B-Air too long to get there, and D-Air will almost never hit. Repeat this process if Luma comes back and Rosalina uses Luma Shot.
-If Rosalina is close to you, and so is Luma, use U-Smash to get Luma away, and then hit the Luma. You might even combo Rosalina along the way.
-Once Rosalina's without a Luma, make sure to take the advantage and go at her with attacks and punishes ready. If she uses the dash attack, make sure to shield or perfect shield it into a proper F-Tilt or U-Smash punish. If Rosalina comes in at you, use the U-Smash mind game, in which you use one U-Smash, then either a U-Smash, F-Smash or D-Smash. This will trick any opponent into running into the second smash attack if they don't have the best reflexes.
-Having trouble with Rosalina's aerials knocking you upward? You can simply destroy her with a well-timed D-Spec; one of two things will happen. One, your D-Spec will meteor her. If she's off the stage edge and if she's unlucky, she'll land below the stage, and it will make it virtually impossible for her to recover due to the angle and direction she's pointing at and distance of her recovery, and the fact that you can punish the recovery as well. The next thing that may happen is that D-Spec will not meteor, but send her upward into a KO. She does have the size of Ganondorf and a very light weight, so KO'ing her from the top would be easy, and no DI can help her out.
-What if Rosalina air dodges the D-Spec? Well, you will either land on the ground or end the D-Spec beforehand, leading up to multiple punishes or even aerial attacks on Rosalina.
-What if Rosalina has Luma in the air? Well, a good F-Air or D-Spec should do the trick, horizontally or vertically.
-Now since Rosalina has a slow, predictable recovery with great distance, you would need to read the recovery to see whether or not you should punish on stage, or meteor. If she goes lower than the stage, meteor before she grabs the edge. If you're around her, punish with a meteor or F-Air depending on angle. If she comes a bit near or above the edge, use D-Tilt.
-If you really want to get good reads in, make sure that if she's far away and hasn't used the recovery, use the U-Tilt, as it will provide a good KO method and edge-guarding method. If she goes well above your head, either run to her and U-Smash or if she's just next to you, use F-Smash or Warlock Punch (before she lands).
-What if she wants to gimp your recovery from below? Bait her out and use a stage spike on her. She will always be pointed opposite of the stage and will be unlikely to recover.
-What if from the side? F-Air, always use F-Air, or if you're pointed backward, B-Air. Even if you can't recover, it will benefit by damaging her. But if you and her are near, use Flame Choke, especially on last stock.
-In this, these are just tools to help Ganondorf get into a better MU with Rosalina, but the best thing to always use with Ganondorf against Rosalina is U-Smash, as it will universally give Ganondorf a helping hand in this MU. It will lead into mind games, combos and more, and it will destroy Rosalina's Luma quickly.
Now onto Sheik things:
That or just Stage Spiking the opponent. The Gravity Grenade can stage spike.
Well, with any character, her best kill moves are pretty hard to land upon, examples being Jiggs and Dedede. There are even some characters who will outright punish her for trying to land a kill move on them, the biggest examples being Diddy, Fox and Link with their high-damaging attacks and punishing combos.