Thank you very much for the fast feedback. I certainly understood what you said, and it will certainly come in handy. Just a few things that will improve the situation a bit:

I'm positive reflect doesn't do anything, actually. Her zombie-inflicting attack isn't really magic, and she can still use it to its full effect. I spent most of my level four spheres on Ultima, and used black magic spheres to give it to other characters. I end up having to use all my ethers, and Yuna's the only real damage dealer with Ultima. Also, when she uses Mind Blast, it has sometimes been enough to kill Rikku and Auron, therefore rendering my party totally useless against Mega Death.

Nonetheless, that was excellent advice. I think I'll give it a try tonight and see if I can finally beat that evil woman!

Cheers.