I'd say Tatsuya could win this mid diff. His Demon Right may not be able to kill Kei. But it should be enough to incapacitate her for a while. And if she come back, Tatsuya may aim for her weapon instead since attacking Kei herself is useless which in turn will kill her instead
Perhaps Elemental Sight can provide that information? I'm not particularly well-versed in the series, so this is only conjecture.
Regardless, Tatsuya can't exactly put down Kei his normal way and he tends to leave weapons and such untouched. So if Kei can take him by surprise, she can win this with soul hax, but there's also nothing stopping Tatsuya from repeatedly blasting her into dust aside from her own resistance to magic (if that would change anything, since magic in Tatsuya's series is more akin to Information Manipulation).
but how will he know her weapon is = to her life?
If turning her to dust over and over again turned out to be worthless thing to do. At least he can disarm her to reduce her options of attacks or even make her unable to go full offensive in this case. However, this decision of his may become a really lucky and nice decision to make since its her Ahnenerbe which contain her soul. Yes he wouldn't know about the whole Ahnenerbe thing, but that doesn't mean that destroying her weapon in order to disarm her isn't a viable option. In this particular case however, destroying her weapon would become the most effective way to defeat Kei.
They're bloodlusted. Which mean they'll use the fastest and most efficient way to defeat their opponent. And what will Kei do to prevent that from happening ? Demon Right works even on stuff like gases, intagible things, signal, magic, etc. Not to mention his range is far more superior compared to Kei. He could keep their distance constantly since speed is equalized. Basically Tatsuya's means of defeating Kei is superior than Kei's means of defending against him let alone defeating him. The result could be changed if the starting distance is closer, speed wasn't equalized, and/or their mind state is in-character. But that's not what the OP says