B-Skill and C-Skill for Saizo

Watersweep, Guard, or Cancel Affinity in B and Savage Blow for C.

Since you have CC, Quick-Reposte would also work, as well as Threaten or Buff for his C-Slot if you want a purely enemy focused build.

Depends on what you want him to do for you team.
Im guessing that your Saizo would act as a debuff/special proc mine to hamper whatever attacks him. So quick riposte makes most sense, its not fancy or original but does the job.. I dont think he has enough attack to use vantage either. C slot really depends on the team, Savage is ok as a standalone but since the point is to have him be attacked that doesnt really have synergy