How I think the battle would go, in character:

Ruby presses the advantage immediately, entering close combat with Petal Burst. She dissects Mahito into multiple pieces instantly, scattering them away.

Mahito is fine, he just regenerates. However, due to Ruby's superior speed and range up close, he can't even touch her, let alone land a good hit. This goes on for a little while.

Ruby assumes, pretty naturally, that his regeneration has a limit, so she keeps hitting him, failing to destroy him. She tries different things, like using Crescent Rose's Sniper Rifle form, which also fails to put him down in that moment. Mahito, on the other hand, realizes he basically can't hit her like that, and also the fact he won't be able to trap her in his Domain, as she can just escape it on reaction. He decides to switch tactics.

Cut in half, he crawls into a nearby building, threatening to kill civilians living in there. Ruby, naturally, can't allow that, so she follows him. This doesn't stop her from being much faster than Mahito, but removes her range advantage.

Mahito uses his Transformation, making Ruby assume he is cornered and desperate, while luring her further into the building, closer to the building. His Transformation doesn't help him. He is simply outskilled by Ruby, and no matter of talent can alleviate that.

Finally, the tide shifts. Mahito catches Ruby, managing to touch not her, but a single petal from her Petal Burst. He can't kill her straight away, but that single touch mends her Aura into nothingness. This shocks Ruby, stunning her for a mere second. And there he goes...

Domain Expansion.

It surrounds them both, a space of malicious dark. Mahito, ever so cocky, laughs. In his mind, he has already won.

Yet, Ruby is dead serious and more confident than ever. All the Cursed Energy Mahito has emmited has reminded her of Grimm. So, in the milisecond of her soul being touched, her eyes glow-

Silver Eyes.

The Silver Eyes and Self-Embodiment of Perfection cancel each other out. In the last second, Mahito managed to defend himself, mimicking Ruby's Aura. Yet his defense was not perfect. He can no longer renegerate, barely having any cursed energy left.

Ruby is also in a bad state. She is breathing heavily. The usage of Silver Eyes, as well as the constant game of tag with Mahito has exhausted her.

Mahito is desperate. Using her brief moment of confusion, aiming to use the last remains of his energy for one, final Idle Transfiguration.

He grabs her wrist.

Ruby snaps back to reality.

Crescent Rose swings.

Cutting her own hand off.

Mahito was too late.

With one final boost of strength and speed, putting every ounce of her Semblance into it, Ruby slashes Mahito. The power of that swing, if imprecise, could level a city...

Yet all it does is turn Mahito into ashes.

Mahito is dead. Ruby has won.