Create (or find) a weapon with "first attack" ability for ANY of your characters and equip it. If the weapon is not Yuna's, then she should NOT be in first free formation to start the battle. When battle starts, character with that weapon gets first turn no matter what. If it's Yuna - fine, if not, just change that character for Yuna. this way you ensure that Yuna gets the first strike. (personally I have Auron with this ability, and replace him with Yuna).
When Yojimbo is summoned, give him money and pray he does the right thing. That only depends on how good you treat him and a little random factor. "Prepare" him for the battle, summon him a lot in small battles, pay him nice money and it should work. BUT, if he does Zanmato attack for 10 Gil against some random battle attack, he won't against the tough enemy. The tougher the enemy, the more you have to pay.
Beware though, the rumour is that you have to beat him 5 consecutive times, that is: you have to be rich.
Finally, when Dark Yojimbo is summoned, you do not HAVE to fight him. He doesn't attack you first. If you want to escape, just use the teleporter. Good luck. Dark Yojimbo is the last Dark Aeon still standing the ground in my game. <img src="/images/graemlins/smile.gif" alt="" />
