Paranoid hit the nail on the head. This is Dark Pit's only significantly advantageous matchup compared to his vanilla twin. Electroshock Arm will murder your squishy friend and ensure the rest of the match will - more or less - come down to fundamentals.
Fortunately for you fine fellows, I think Rosalina still have the advantage here if she plays cleverly. Her aerials have great range and the disjoints can trade favourably with Dark Pit's, and her meager weight makes it relatively easy to escape from his combos. The issue is that Dark Pit is great at racking up damage and that weight will come back to bite you in the keister if he gets a read in on you: one good smash at high percents is usually enough to close the stock.
Dark Pit is great at conditioning. He has a response to every situation and will condition you into reacting in a certain way, then punishing you for it. This means you have to play judiciously and mix up your tactics if he gets the read. Dark Pit has the better grab game here in my opinion, as d-throw sets up for a host of different moves and, in some cases, can even set up for a kill confirm (d-throw -> b-air or f-throw at the edge - remember these!) You should be able to space him with f-air and b-air, u-air and d-air will keep him zoned out if he's in the air quite frequently or trying to catch you in the air. I think Dark Pit has a tiny bit more muscle than Rosalina, which can make him put the pressure on well, though your smashes have great frame data and decent KO power and it's a good idea to keep them fresh in case you corner Pittoo and need to close the stock.
If Luma is still there with you, hold him off and soften him up before going in for the kill. If you seal off his avenues of approach (on foot or in the air), it will shut down Dark Pit's main tools of offense and make it difficult to get near to you. This is why he will tend to target Luma first to force the match into a game of footsies. Do note that he can reflect Luma with Guardian Orbitars. This is a good why to get the kill if you're being hasty in the fight.
Off-stage, Pittoo has a noteworthy weakness compared to Pitty-Pat: his arrows travel straight rather than in the free formation of Pit's. This means he can't really snipe you off-stage compared to Pit, and instead will have to go deep for the kill. This is dangerous if your aerials are on par, as u-air can scupper Pittoo's chase-down for a d-air spike, while you should be able to trade with f-air and b-air if he tries to get you out of the blastzone with them. This applies in reverse too: d-air is great to punish a readable recovery: Dark Pit can either go deep under the stage and then recover upwards with up-b, or he can dash horizontally with side-b to catch the ledge. The former has no hitbox and can be intercepted, the latter has super armour and is invulnerable while traveling. Your recovery is much the same, of course, so we both want to mix it up and make it difficult to get the read in.
If Luma is valid for the mixups, go for a jab combo at the edge. It will make Dark Pit's recovery options very dangerous and can force him into a position where he can be d-aired easily.
I'm gonna say this matchup is distinctly easier for Dark Pit due to the fact that Luma is much easier to deal with and his fundamentals are very good for one-on-one fights. That said, it's still not the easiest matchup in the world and Rosie has more than a few tricks to keep us at bay even with our incredible footsies, so I'm gonna mark it up with that in mind.
Matchup score:
±0
EDIT: With customs on, Dark Pit has access to Guiding Bow, which makes it easier to maneuver the arrows around Luma and hit Rosalina, while your Luma Warp can make it much more difficult for us to get the hit in on Luma. It's still very possible to read where Luma will end up and KO it then, of course, and the arrows do make it much easier for Dark Pit in the grand scheme of things, so I think the matchup remains the same.
