First to get the dark Knight Dressphere. You should've gotten it in the lowest deepest darkest dungeon of Bevelle. Before you faced Ballairi or whatever his name is. And before Bahumut.

You can beat Yojimbo by not using a black mage but by having a white mage or alchemist (potion plus high potion makes mega-potion) dark knights. First cast a chocobo wing or a chcobo feather. Id use a chocobo wing. You just got to keep your HP up. And fight, dont worry if more than one of your characters are dead. Use a mega pheonix down. When your down to 1HP and 1 MP....Dont count on MP anymore, ya have to get your alchemist or white mage to hurry up and heal everyone. Might want to have a grid sphere that can block out posin.