LEVEL 99!!! You definately need to get to that. Also, MASTERED MATERIA EVERYTHING. No way you'll be able to defeat Ruby w/out a super powered character, which would be Cloud. Keep him alive and protected and get those OmniShlashes IN! Knights of the Round do help a lot too. Plenty of Elixers/Ethers/Megalixers/ whatever else that would help you. It's a tought battle, just try to max out your best characters and attack like crazy.