I didn't use Yojimbo either, what the hell, so much rules? I did succeed once using Zanmato on Dark Bahamut but I think... it's not fun at all so I reset. As long as you train hard, you can kill every fiends even Penance. I'm training hard now, My Wakka and Tidus already finished the entire sphere grid, I'm going to train Rikku, Lulu and Kimahri since they're the weakest. Dark Aeons is easier than I thought, I just killed Dark Valefor, Dark Ifrit, Dark Ixion x2 and Dark Shiva. Omega Weapon is a joke.