My Rikku has her sphere grid full and does the max damage amount of 99999 with break damage limit. All of my characters that I use have break damage/HP limit. Yuna's the only one who does lower than that and she does about 70000. Rikku filled up her sphere grid first, then Tidus, and Yuna's being really slow with it. It took me about 200 hours as well and Yuna's the only one without it complete.
Shoden, check out www.gamefaqs.com you should be able to find a faq to help you get dark matter.