Zanmato's definitely the way to go with Dark Aeons, except for Dark Yojimbo who you must fight five times in a row. No-one's that lucky with Zanmato.

Zanmato comes up more easily if you're nice to Yojimbo. Never let him get defeated, always pay him plenty - basically, if he uses Daigoro or gets dismissed penniless, you've gone down in his ratings.
So, be nice to him, then, when fighting the Dark Aeons, do this:

Have a character in the party with 'Initiative' ability.
When the battle starts, switch that character with Yuna, whose Overdrive gauge must be filled; Grand Summon Yojimbo, and pay him heaps - I always used 77,777 Gil, and it resulted in Zanmato about one third of the time. If Zanmato fails, re-set and try again unless you're strong enough to win without it.