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I remember the narrator said as ZWJ entered the Brightness Peak battle late on that had Yang Xiao or Wei Yixiao not been seriously injured, they would've had no hesistation about taking on Miejue with the Yitian sword. In fact, it was them that showed ZWJ how to defeat Miejue.
Besides, the OP was talking about assasination, not duel. I'm pretty sure that Yang Xiao wouldn't be dumb enough to ask for a one-on-one duel with any of those. The weaker won't show up and he'll be asking for unnecessary troubles with the stronger. Yang Xiao would find a way to steal the Yitian Sword and after that, I think he'd have no problem sorting out Miejue.
"Anything you can't say NO to is your MASTER, and you are its SLAVE."
"I disapprove of what I say, but I will defend to the death my right to say it."
The narrator just said that if Yang Xiao and Wei Yixiao weren't injured with their martial arts, they could have fought Miejue themselves. And they MAY not lose to her. It was more of a comment on how comparable the martial arts levels of these three experts were.
So huge, so hopeless, to conceive
As these that twice befell
Parting is all we know of heaven
And all we need of hell.
Emily Dickinson (1830-1886)
Sure, but the fact is that they did know the right strategy to defeat Miejue with the Yitian sword. There was never any indication to suggest that Miejue knew how to deal with Yang Xiao or Wei Yixiao. I'd bet on the Ming Cult guys.
"Anything you can't say NO to is your MASTER, and you are its SLAVE."
"I disapprove of what I say, but I will defend to the death my right to say it."
I'd take Yang Xiao over Miejue, but I'd take Miejue over Wei Yixiao. (with sword)