Dark Shiva is horrible to kill. I equiped Yuna with a First Strike weapon so she could summon Yojimbo before Dark Shiva killed us all. But I tried about six times and not once did Yojimbo use Zanmato. It was then I realised I didn't have enough Gil.

Off-topic: Could anyone tell me how to get the Celestial Mirror? I have the cloudy mirror, but I don't know how to turn it into the Celestial one. I sincerely hope I don't have to do that butterfly quest, I'm REALLY not good at that.