Astner
The Ghost Who Walks
Byuakuya vs. Sasori
[list][*]Buyakuya from the beginning of the Soul Society arc.
[*]Sasori from the beginning of the Kazekage Rescue arc.[/list]
Assumptions: [list=1][*]Reiatsu = Chakra.
[*]In-character.
[*]No beforehand knowledge of the opponent they're facing.[/list]