If Sasuke keeps his distance and just destroyed any Mokuton variants with his Legged Susano'o, he can also start to ignite various parts of the area in Enton, to begin prepping for Kirin. Mokujin is the only thing in Base Hashirama's arsenal that can remotely compete with a CS/kyuubi Cloak enhanced Susano'o, as it was capable of effortlessly cutting a giant Shinju branch prior to Senjutsu enhancements. Jukai Koutan is hardly helping Hashirama in his offensive.
Sasuke can deal with Flower Tree World by igniting it with Amaterasu, and or utilizing his Hawk summon. Bringer of Darkness is not a threat, as Sasuke would see through it. Since Sasuke has the Hawk, it can allow him to evade the majority of Hashirama's arsenal, until he comes back down. Keep in mind Sasuke also has Aoda who can play a great support role and or he may possibly be able to coat a portion of him with Susano'o.
In the end of it all, once Sasuke whips out PS, Hashirama must use SM as nothing in his base arsenal is capable of competing with it. It's restricted in this case, so Hashirama dies rather quickly. By this time, I'd assume Kirin would have been prepped earlier in the fight.
Edit: Hobi might be able to protect Hashirama from Kirin, but repeated PS slashes will undoubtedly destroy it.