Genryūsai Shigekuni Yamamoto vs the other Captains of the Shinigami.
Battle takes place in an abandoned Soul Society
No prep, morals on.
Round 1: The Captains as of the beginning of Bleach
Round 2: The Captains after Aizen's betrayal
Bonus: Assuming Yamamoto cannot defeat all other Captains, what would you say is the largest combination he can defeat at one time.