agreeing with Gerbill.
To level up, just get into fights between the 2 save spheres in the dome of the zanarkand ruins. It's an awesome place to level up.
Once you think you're ready to take on Yunalesca, know that she has 3 forms (I can't remember all the HPs).
1. When you attack, she'll inflict darkness. When you cast magic, she'll silence you, etc. etc. Don't use aeons yet.
2. Have your holy water/remedies ready cus you'll need them a lot. Just hack and cure, hack and remedy etc. When her second form is almost done, it would behoove you to keep anyone with the zombie status that way. I hope you didn't use aeons for this form, if so..be patient.
3. For her first attack, I guarantee it'll kill anyone who's not a zombie. Let your zombified characters remain that way, and if she tries Regen on that, just dispel it. Now it's aeon time.
I'm not sure if she's immune to the zombie status herself, but find out. <img src="/images/graemlins/wink.gif" alt="" />