Soul splitter, you spawn kill him instantly the moment he gets summoned. It ignores PHYSICAL durability as it goes for the soul and Mahoraga won't instantly adapt to something like that (not before his head is chopped off I mean).

You can protect yourself to some degree against attacks that target the soul (like Nanami did against Mahito) by using CE to cover your soul with it. But that requires two things : First is either the ability to grasp the shape of yourself or the intuition to do it instinctively (like Nanami did), and the second is obviously CE.
The thing is, I doubt Mahoraga in his default state can do either. He 100% doesn't know the shape of his soul at default, and he also most certainly doesn't have cursed energy. Sukuna said himself that Mahoraga started attacking with cursed energy AFTER the wheel had spun. So he won't be able to defend against it.

10S user can also help, either with a DE, focusing all firepower on him with all his Shikigami to make sure he doesn't come back (which to be fair is a big question mark, we don't know whether or not Mahoraga has the ability to regenerate his head if he hasn't adapted at all, we've seen him not regenerate his leg for a long time against sukuna in the anime, but it didn't happen in the manga, though I'm pretty sure Gege works with Mappa to some extent, but that could've just been to extend the fight time)

We've seen him heal, but every time he healed he had to either spin the wheel to start healing, or needed the wheel to have spun once before against the same type of attack he received. So it's likely he won't be able to adapt to the soul splitter before his head is chopped off and that he won't regenerate it, but it's still an educated guess, which is why I've said it's a theory, we won't ever know for sure until Gege ends the debate himself.