Which it can’t see through a thick poison fog, which is also spreading up towards the bird so.
Aaaaaaand that's why it has Wind release. Before the poison spreads to its summoner it's a rather simple matter for both it and Baiyan to coordinate in a wind-release to keep the clouds at Bay. After which Baiyan can choose to escape underground, and unpoisoned, and from there a combination of utilizing that genjutsu I set up, attacks from below, and aerial assault is more than enough to trouble Reaver. Even better he'll be isolated from Azumi because she can't effectively fight Baiyan given the state of the battlefield.
It's only a matter of punching a hole in his suit with a technique, as Sagisō has done before, and letting his own poison do the work. After that, she can focus on assaulting Azumi from 3 different directions at once.
Of course, that's the most Idea scenario, I have many more alternatives that achieve similar results. One alternative requires purely earth release, but I don't want to use that one as it could be put to far better use once Azumi's a bit closer.