Alright, I'm here, gonna try to explain this best I can. I tried to get the opinion of other Charizard players, use my Rosa against them, and get an overall feel of how the pros and cons work out how this match up works.
The unfortunate is that Rosa has some amazing representation in the competitive scene. Charizard on the other hand is still a bit on the unrepresent. As Silentdoom has already mentioned it's really difficult to know how the matchup would work at high-top level play and the most I can get is what happens on my level (and what I will assume most people who use Smashboards are on)
The fortunate is where Rosa tends to make a lot of fat characters struggle within the roster, Zard is probably one of Rosa's biggest challenges (no pun intended) This is mainly due to some tools Zard has that are oddly appropriate for dealing with Rosa
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What you should be thinking
How your mindset should be: Zard is strong, but don't let it go to your head. It's tempting to try and finish her off quickly due to how light she is, but you have to be patient or you'll find yourself at a disadvantage rather quickly.
How the match is going to be played: There are three things to know about Rosaluma that you should keep in mind which is the areas she struggles with. The obvious is that without Luma everything about her significantly drops. The less obvious is that her recovery is not really that great. But one of the most important things to remember and smaller details about Rosa MU is that Rosa is not really good at handling stage pressure. This being, the less room that Rosa has on the stage, the easier
Ideally, you want this to turn into a Sumo match where you try to push Rosa closer and closer to the edge of the stage and punish any attempt to relieve pressure. Don't concern yourself with how much damage your doing to Rosa, but rather how much space she currently has.
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Neutral
What you want to use during neutral is Flamethrower, Rock-smash, Auto cancel F-air and N-air, and jab 1 & 2. A lot of the moves not mentioned such as Zard's tilts can still be used. Try to use them sparingly though to simply remind her that you have those moves; but if she catches them whiffed she'll be able to punish you for it.
Flamethrower is amazing in this MU. I keep telling people, Rosa can shut down projectiles if she can react to them with gravitational pull but it's a different story if they're used at a shorter distance. In fact, you can condition Rosa into using gravitational pull which means a free punish if you 'act' like you're about to FT her. In the case that FT is GPed, then immediately let go of the special button in reaction when GP comes out. GP doesn't have enough lag to punish out of FT but it's possible to apply pressure with one or two Jabs (if she's in that proximity and you wished) But if not, just respect it. No need to fear, you're not committed enough out of FT for her to punish when GP is off.
Some Rosa players will choose not to use GP and simply shield flamethrower. If this is the case, then use it to keep her shield in constant check. The less shield she has available, the more danger Luma is in from pressure. Otherwise flamethrower will almost always be considered a good option if you catch, well, almost anything on whiff for free damage on both her and Luma which is great. But despite how useful flamethrower is, don't get too predictable with it just like every character would. She'll use an aerial option or cross you up and get a free punish which you'll regret your decisions that moment you loose stage control.
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Mix-ups
So what's the answer to keeping you're flamethrower both fresh and unpredictable? Apply pressure with lagless, or moves with very little lag. Play the match in Rosa's face, but keep reserved and away from her tilt range until you gain an advantage. Mix up what you use, and when you use them. Short hop F-air is pretty helpful. In some cases where you catch a whiffed jab, you can actually directly knock Luma out of the picture (You can also answer whiffed jabs with a tilt). If she see's you doing this too often though she'll challenge with U-smash which can easily launch you into the most unfavorable area. When you predict her doing that, go for a Rock Smash to challenge U-smash (Rock smash is the go to for any read smash Attack Rosa might use) When using F-air auto cancel for shield pressure mix up where you land to either space for a jab or get out of her tilt range.
She'll try to fortress herself with a mix of her tilts, she will win the spacing game due to her frame data being much better then Zard so try to avoid her tilting range.
Mix up your movement patterns and keep close for each time you apply more shield pressure to her, the more she's going to have to give up her space placing her to more stage pressure.
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She's pinned! Now what?
In order to relieve the stage pressure you create, she's most likely to do two things that are unsafe. During the match, analyse her behavior, read, and act accordingly. Standard roll cross up, punish with however you enjoy punishing a roll. Something you've been training on in For Glory right? No different when facing Rosa
(I would personally grab her and throw her off the stage) The other option she'll use to try and reset neutral is short hop N-air. She can use other methods but most are foolish. If she reacts to your approaches with N-air, then shield the first hit (Luma hit,) and if she decides to try and fly over you, OOS U-smash (there you go, free U-Smash and Zard is a pretty good juggler, Rosa is pretty floaty as well) You can even go for OOS fly if you want.
During this state, you'll also be able to get her off the stage at times. Her recovery is OK, but it's not really that good. Rosa will use a mix up of it to either lock herself back on the stage's ledge or try to send herself to land near the other side of the stage. Perfect for Zard to capitalize on, it's got no hurtboxes so take advantage of this situation here. Practice Zard's offstage game, it's very suited and will benefit you while fighting Rosa.
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She's fighting back! Well of course she is.
She's probably going to do what she does best and her best option is a utilization of her tilts and combos to pop Zard into the air as much as she can. Pay attention to her behavior and avoid the situations she can place you in. Every Rosa plays differently as she is a rather versatile character. I can't tell you what the intentions are going to be and it's up to you to find some players to practice with so you can get familiar with her options. Just don't go into autopilot and keep a close eye on her behaviors, Zard does have an answer to most of what Rosa can do. But you can't simply react, you gotta work towards the Rosa's behavior.
And please, don't get sandwiched by Luma during Neutral, and especially don't jump into the sandwich. Very bad things happen in there. I hate it when people jump into my sandwich because I feel kind of bad since they don't know how to fight Rosa. If Rosa is desynced and pressuring you with Luma from a range. Judge the range and don't make a commitment that will cause Rosa to go in and punish, usually, you'll find yourself flamethrowering Luma. But depending how far Rosa is, you can use something like a rock smash, depending on the situation.
Just always keep in mind, the more space she gets, the more options she gets. You keep that in mind the more you can judge and respect the disadvantage your at
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Wait, who controls the pace?
The short answer, neither of them. With the amount of time I placed into both these characters, I will easily say that reckless behaviors are going to cost them severely in the long run. They both have to play patiently and defensively until they recognize when they're at advantage and go for bigger commitments. But in the chance you get one of those types of Rosa players who would play recklessly into you then why not? She's asking for it.
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Conclusion
These are both primarily defense characters. Rosa has the same advantage she does with most heavies having better frame data, tools, and being able to pressure them from a range in which they have no answer to. However, Zard has something that many heavies lack, mid-range pressure that's not connected to his hip, juggle capabilities and an existent offstage game. To make the point clear, let's make a comparison with DK. What's he going to do to Rosa? Rosa keeps him pressured with no need to approach, DK needs to use something that's going to be unsafe, then Rosa punishes and places DK into an unsafe position.
Zard will still be juggled like the other heavies as well as have trouble recovering if he makes a mistake, but he will be able to fight back if he's on his two feet. The MU is going to go down to who can keep the stage control and who is making the smarter decisions during neutral.
50:50
If anything else, very slightly advantaged in Rosa's favor.
Be sure to practice against a verity of us Rosa mains to see what we do and lab your options. The more Rosaluma mains you fight, the more you learn this MU and the less you gotta worry about
I'll be sure to leave some additional notes about the character later. Just a couple of minor, yet important details of Rosaluma's properties which is pretty good knowledge of having. But since this is about Zard against Rosa, I'll leave the post like this.