(1) Boa is not "YC" class. She has advanced Hakis, CoC, and a hax DF.
(2) You need to be Admiral level to beat her 1 v. 1. Koby is nowhere near Admiral level. Beating Boa is an EOS goal for Koby.
Imagine thinking a guy who is 2 ranks away from Vice Admiral is going to beat someone who creams every single VIce Admiral.
This the strongest VA and the head of Marineford right now.
If Boa gets defeated it'll be a team including an Admiral. Koby will be the water boy at best.