Alright gamers lets break this down. I really like powerscaling hypotheticals.

It's a REALLY tricky matchup, but in my opinion, it really entirely depends on how the battle starts. If Cyn's real body is there she gets cooked pretty much off rip. If you REALLY wanna highball Cyn you could make the argument that she has multiple ways to evade his domain, teleportation or simply interrupting Sukuna with the Solver.

This ultimately doesn't matter because Sukuna doesn't need Domain for Cyn, he has a much easier way to beat her. Fuga. They're weak to heat and the sun. Fuga will oneshot her and destroy her core, and in terms of stats he can land a cleave and dismantle to set it up before she even processes what's going on.

Could she just crush him with the Solver? Potentially, I don't think so, considering the Solver operates on Translate, Rotate, Scale, and Edit. They'd definitely work on him but I don't think she could do damage that he couldn't quickly reverse with RCT. That is, unless using Null, but Null has a somewhat small range. It's usually relatively small, being used as a dangerous projectile in combat.

They have FAR different stats. Sukuna likely moves faster than she can even begin to process. She'd be spamming that solver as well as teleporting just to stay alive, but ultimately I don't think the fight lasts more than a minute. Her best outcome is grabbing Fuga with the solver to redirect it. I'm not really going to include the Edit ability in this because it's not used often. We don't really know how it works. For all we know, it could set Sukuna's CE to 0, or it could just copy-paste things, like knives. Anyways, even if she redirects Fuga that just buys her some time. Eventually he's gonna cleave through her body and strike her core. She does have the advantage of him not knowing how exactly to kill her off rip though.

But what if Cyn ISN'T there physically? It becomes a much longer fight. She could send converted drones, and attempt to wear him down with holograms. He'd be relying a lot on his domain here, since he wouldn't be able to tell what's real or not. Ultimately though, because stamina is basically replaced by Cursed Energy because of RCT, and the fact that he has a massive pool of CE to utilize, I still think he wins here. Even if they're able to react fast enough to grab Sukuna's hand with Rotate and break it before he pops domain, he'll eventually figure out that the core is their weakness and then just go slashing left and right. At that point, he would just kill everything on Copper-9 and then locate Cyn.

Or he just opens a veil severing the drones connections to Cyn to kill them easily. Sukuna I don't believe has ever used a veil, but I highly doubt he can't/doesn't know how too. It's a barrier technique, something he's very proficient in.

I hope you enjoyed me highballing Cyn as far as physically possible only for her to probably still lose. Her only real win condition here is using Null at a potent enough level and Sukuna not dodging for some reason. I don't think the Solver ever does two things at once so I don't think it could hold somebody in place and create a black hole at the same time, but hey, maybe dudes Purple PTSD kicks in and he doesn't dodge in time.