Kakashi would need knowledge before hand that beneath Susanoo is the weak spot. Using bunshin feats or smoke bombs at this point(similar to what he did vs Nagato) would give him the opening to travel underground and get to Sasuke.. That's the only way he can beat Susanoo, and that's if Sasuke isn't bloodlusted and doesn't just start spamming the hell out of susanoo arrows before Kakashi even has a chance to do anything.
I wonder how using kamui on Susanoo would work. Would it just warp the susanoo, or would it take Sasuke with it? Given Susanoo is a guardian of the user, I would assume Susanoo would have to be warped prior to getting Sasuke, and he would have to instantly warp susanoo, which would leave him vulnerable to Sasuke, going off of my theory on how Kamui works. And using kamui right off the bat is completely OOC for Kakashi, so that really wouldn't even matter anyway.