[FFnut]

Get command counter materia, and grow it until you have eight mastered. Do the same with mime. Have one of your characters with eight sets of linked slots so you can set up an eight-linked counter-mime chain. Give your person as much weapon power as possible and get limit (preferably one with multiple atacks). Launch a limit once the battle starts and don't do anything. Once Ruby attacks the character who used limit and has the insane amounts of counter-mime...

Well, I've seen this combo beat Emmy in a minute and eighteen seconds with Barret's Ungarmax. If it can beat Emerald that quickly, imagine what it can do against Ruby.