Yea, that's why I level Shadow to 99 while I let all the others stay at 40-50.

From what I've heard it seems like most people beat the Dark Aeons by using Zanmato. However, I think it's cheap to use Zanmato on bosses. Anyway, Zanmato doesn't only depend on luck. I think there's some tread around here that explains what to do to make him use Zanmato more often.