Being a main of both characters I think that I should say what I know about this match-up
First off and most important thing:
Did you remember the Olimar match-up in Brawl days?
Well, at a point I thought that this match-up was like this.
But let's start off with the good things.
- Ganondorf can K.O. Her very easily, usually under 100%, prior to her being very light, especially to the upper blast line(prior to her being floaty);
- Ganondorf's attacks usually outprioritize hers when she's without Luma, of course, assuming her attack isn't trascendental, but with Luma things get weird since, if you didn't know, Luma is keen to wreck every rule of priority.
Well, that's about it.
When Rosalina sends away Luma you'll want to use USmash, DTilt and hitlag/auto-cancelled aerials, since other attacks will most likely get punished.
With Ganondorf's high falling speed and big hurtbox, she can set-up a jab reset or a Bair/Nair lock with the Nair and Fair wich will end with a FSmash and a subsequent gimp if you aren't ready to tech.
Additionally, she can juggle Ganondorf 25/24h.
She also can end a stock pretty early with various gimps, namely with her Dair for Dark Dive and Bair for the Flame Choke (speaking of this, as it has already been stated, Ganondorf might end up frametrapping himself if he lands a Flame Choke) and by sending Luma to the ledge or offstage with smash attacks other than the afermentioned Dair and Bair(she needs to call Luma back for it to send you offstage).
She can pressure Ganon's shield heavily with Lunar Landing(auto-cancelling early her aerials, usually the Nair, so that she'll be lagless or nearly while Luma attacks, if she uses Side B or Neutral B Luma will partecipate normally to the attack interrupting the afermentioned aerials), with jab, star bits or by sending Luma near you and button mashing, as such there's no pratical option other than rolling away, granted you don't powershield the attack.
Did I forget something?
Oh, sure.
If you didn't know, Luma can attack only in the helpless state of Launch star, wich means that, after juggling you, she can UThrow/Smash/Tilt->Launch Star->Shoryuken(Luma's Uair) to kill early off the top blast line just as well as the bottom and sideways one.
But there are two situations where you are at an advantage.
The first is while juggling her, she can have trouble avoiding an Uair without reading it, if you for some reason menage to grab her, you can DThrow->Uair her and position yourself fiagonnally down in front of her, a position that can be covered only by the late hit of her Nair, Luma's Dair can hit Ganondorf as well, but the attack is slower than the Uair, just be aware of the Luma when he's on the way back to Rosalina.
The other situation is where she is recovering.
Her recovery has no hitboxes and can only travel three distances, despite traveling fast and far, giving you a chance to finish her stock.
It's not like I think this match-uo should be a straight 0:100, but she is at advantage in neutral and at an even harder one for any situation after the neutral, with the advantageus situations for Ganon being hard to pull off because of the afermentioned trouble in neutral.
However, with custom moves enabled, this match-up might become somewhat playable, though it probably still isn't even.
You will want to use the most common set(2-3-2-2) and here's why.
Warlock Blade
It can eat all of Luma's options when sent out, forcing Rosalina to call him back, with his deceptive range potentially making it unpunishable, it can also pressure a distant lunar landing, and will also blast away Luma, also if it breaks her shield, Warlock can finish her reliably.
Flame Chain
Other than not frametrapping Ganondorf, it'll also drag Luma offstage, though it's likely to be punished if Rosalina shields(unless she's on the ledge) or dodges the attack in various ways.
Wizard Dropkick
Helped by Rosalina's sub-par aerial speed, it can save Ganon from juggles, other than making his recovery a little scarier and better, forcing her juggling to be at ground level
Dark Fists
It can eat the hit attempting to juggle, edgeguard or shield pressure you(unless it ends with a grab) and can be used as mix-up and with baiting in general akin to Vanish, since the Super Armor lasts until the hitbox comes out (and I'm pretty sure that that hitbox is trascendental?), but it kills stupidly early already at a ground level.
For what matters of Rosalina's customs, Shooting Star Bit can be powershielded (if expected) or dodged with Wizard Dropkick, while you have to be cautious with Floaty Star Bits. With Luma Warp you must be more cautios than Luma Shot since the move is nearly istantaneous at the cost of sending Luma a set distance and not being able to kill. I think that Launch Star Plus can help her with the Luma's Shoryouken. Luma Guardian can be used by her if you are too relentless but it will often get beaten by Dark Fists.
[52.5:47.5] 
