fighting Ultimecia is extremely long, I'd suggest you face to omega weapon, this will allow you to learn how to fight in a very long struggle with different attacks, this battle is a way to train yourself to confront Ultimecia. she can use her guardian in different ways and forms, even she can merge with his guardian to gain more power. this fight could last about an hour. Once you get beat omega weapon, fighting Ultimecia will be similar. something interesting in fighting Ultimecia, is that every time you lose a player that will be replaced by another.