Shunsui Kyōraku
Sasuke Uchiha
Rules & Stipulations
This is Eternal Mangekyō Sharingan Sasuke
Shunsui has access to both his Shikai & Bankai
Both characters have basic knowledge on one another
They will both be in their usual mindsets
Victory will he achieved by death, knockout, or incapacitation
The battle will take place in the valley of the end
Having basic knowledge means that Shunsui would consider Sasuke a threath because of Genjutsu (which he most likely compare to Aizen's KS). In that case, Shunsui would get serious quickly and go for the kill, which he could actually achieve with relative ease by just using Kage Oni to teleport into Sasukes's shadow and attack at a point blank, especially considering that Shunsui should have the advantage in speed going by scaling (he was fighting Resurrection Starrk, and just base Starrk was too fast for Ulq Fight Bankai Ichigo to react). If Shunsui uses Bankai then the battle is decided IMO.
Sasuke does have a chance to win tho, and that's by using Genjutsu before anything I wrote above starts to happen.