- First off: Dark Pit is objectively superior in this matchup. If you have to fight Rosie, you should seriously consider using Dark Pit, even if you'd otherwise play Pit. Electroshock makes this matchup significantly more bearable and arrows will seldom be useful in the first place, so he has a noticeable advantage over Pit in this matchup. That said, I'm going to presume (unless otherwise stated) you're playing Dark Pit. I will make a brief point about their respective options at the end, just in case you do decide to go Pit.
- Dark Pit's Electroshock is great in this matchup. It's one of the best guaranteed Luma killers you could hope for and makes things much more even. Your first priority, therefore, should be to kill Luma: irrespective of Rosie's percentage, Electroshock can knock Luma off-stage and send it into a fatal tumble animation from just past the mid-point of most stages. Try to aim for the side that Rosie is closest to and get rid of it as quickly and efficiently as possible. If you're facing the right direction and are in any way close to the edge, Electroshock should knock Luma off-stage in one hit. That gives you about 12 seconds of one-on-one time with Rosie to press home the advantage. Use it wisely, and don't be afraid to go for Electroshock again whenever you need to: just be careful that you don't get too aggressive with it and try to use it as a crossup or mixup.
- You will probably get punished for using Electroshock, at least at low percents. This is something that you will have to deal with, and it's really not a heavy price to pay for negating Rosie's main weapon: if Rosie is shielding, she can grab or punish with an OOS aerial or u-smash, neither of which will really have a noticeable impact at low percents, or she can punish with a dash attack or possibly a smash of her choice if Luma is disconnected from her and she can rush in on time to get the punish. Remember that shielding will not defend Luma, so don't be afraid to eat an OOS punish if that's what it takes. When you get to higher percents, and if Rosie decides to be more careful with how she uses Luma, you will want to weigh up your options and decide if it's best to risk potential punishment to weaken Rosie. You don't have to kill Luma, it's often better to just play the neutral and go for Electroshock without making it a priority, but keep an eye out for opportunities and decide when it's safest to use it. It really does make a difference in this matchup, and the two are much more evenly matched when they're on their own.
- Rosie is no pushover without Luma, remember that! She's still got a ton of disjointed options, reasonably strong smashes and a decent neutral game to keep you on your toes. It's not uncommon for neutral with SoRo to become a patient game of playing careful footsies and trading disjoints, but remember that she is still weakened in this state and you should not be afraid to pressure her with whatever you've got. Space her out with autocanceled aerials and pivots, going for the grab if possible, and don't let yourself get punished. Rosie's still fast and strong even by herself and she is more than capable of punishing you if she gets the chance, so find a good balance between offensive play and patient poking to get her defenses down.
- She may decide to play more defensively and stall the clock if she's by herself, so don't be afraid to pressure her and make her respect your neutral game if she doesn't want to commit to the fight. While she can punish you consistently if she gets the chance, remember that this is your time, so make the best of it!
- Grab is an effective option as always, but Rosie's gravity makes it trickier to get most of our best followups our of throws: d-throw -> u-smash stops working very quickly due to her floatiness, so it may be best to go for d-throw -> u-air instead if you can, and it can be difficult to keep her in combo strings that would work on faster-falling characters. Go for the grab if you can, just remember to mixup your followups to compensate for her stature.
- If possible, don't position yourself directly above her: u-air will make it difficult to land if you're trying to get down from overhead, and especially if she has the advantage, she can keep pressuring you with u-air and potentially take your stock with it. If she gets you overhead and you have to reset, don't airdodge - she will read it and punish you for it, and that can be fatal if you're at high percents or on a stage with a low ceiling. Pittoo's airspeed isn't great either, making it difficult to jump out of the way, so your best bet is to either read her u-air and airdodge at the right time or use Orbitars to descend safely.
- By the same token, it can be difficult to pressure Rosie from beneath (with u-smash or u-air, for example), since d-air is a relatively safe disjoint and can interrupt our punishes. Don't get too hungry for the punish right away, instead try to bait her into committing to something and then punish that instead.
- She has an easier time edgeguarding Pittoo than he has edgeguarding her, since her recovery is about as effective as his and she has bigger disjoints in u-air and d-air. If she gets you off-stage, don't bother trying to fight back: just get back to the stage as efficiently as you can and priorities avoiding unnecessary damage. If you get her off-stage, and because her best aerial disjoints only hit overhead and beneath her, you can trade with her nair and f-air with our own f-air. Don't bother trying to d-air spike her off-stage, her u-air makes it too risky an option. Both her recovery and Pittoo's can be easily traced and intercepted: try to recover in a way that helps you return safely when you're off-stage, and when she's off-stage, cover the ledge to keep her from recovering safely (n-air is a good option, since it has a wide, lingering hitbox that can catch her when she travels). The main crux of this fight will be on-stage, so don't feel too pressured to challenge her off-stage if you're not comfortable with it - just remember that she can stall off-stage to wait for Luma to regenerate, so be ready to challenge her if you have to.
- When Luma is in play, you're going to need to be more defensive and patient until you can find a good time to knock it out. Don't rush in or try to force anything, you'll just get blindsided by Rosie's sentient stonewall: instead, bide your time and pressure from a safe distance, poking with aerials and throwing out arrows to soften it up. Rosie can't commit to much either, since the threat of Electroshock is ever-present, so the neutral can become something of a stalemate where both try to force the other into making the first move. If you're feeling pressured or have a good lead in percentage, it may be pragmatic to just go for the Electroshock kill and force Rosie into disadvantage. Both Rosie and Pittoo have to respect the other in neutral, since they both have a secret weapon that can really threaten the other and potentially neutralise their best options: being able to break through that wall of respect and go for the kill may well be the best thing to do in certain situations, and if you're confident that you can take whatever punishment you may receive for it, Electroshock will be your best friend and the defining trait of your gameplan in this matchup.
- Pit isn't nearly as good a choice in this matchup, since the only notable advantages her has are his arrows and the sweetspot on f-tilt: while f-tilt can be a decent and relatively safe kill option in its own right, it may well have to go through Luma to be effective and it doesn't make up for the option to negate a good deal of Rosie's options whenever the opportunity presents itself. Arrows can be useful for sniping Rosie and racking up damage, due to her tall hitbox and the fact that they can dodge Luma if they're directed well, but Gravitational Pull will absorb most of them if Rosie gets the chance, and again, it's not a worthwhile trade. Upperdash isn't a bad option by any means, and it's still a strong kill option in neutral, but the fact that it sends Luma straight upwards means it won't kill it as effectively as Electroshock and it's about as good as Electroshock is for killing Rosie. There's no reason why you should pick Pit over Pittoo in this matchup if you want to be more effective, Pittoo is easily the better choice.
This is one of the few matchups where there's a significant difference between the two Pits and where you will want to use one over the other in just about every way. For that reason, I think this matchup is
0 for Pit and
+1 for Dark Pit.