If you got Cactuar,use the Kamikaze ability on her.Then she will be beaten with one hit.Thats what I did.

If you don't have either him or that ability use Regen and Curagas on Rinoa,triple on yourself,and Use Blizzaga,Firaga,Thundaga,and flare on Adel and also use your attack command on her if you can do damage thats from 4000-9999.Thats what I did when I beat her in my second Time playing through.

Hope that helped.