it took me around 45 min to beat this guy on my first try so youll have to be patient. have yuna, lulu, and wakka in your party when you start. start with destroying the head first by using lulus fire or if you have any of her 2nd level, use them and wakkas ball on the head, use yunas magic to heal lulu and wakka with their hp and esuna if theyre poisoned or with any other effects they get from the gui. when the head turns grey you had killed the head and you can move on with the boss's body. switch wakka with auoron so you can destroy the arms first becaus they will block the guis body. use aurons skill and lulus fire magic to kill the arms. after the harm the body as much as possible before the arms regenerate. the arms will regenerate after the guis 3rd turn. continue using yuna to heal your party. use lulus fire and auorons skill on the arms. also you can use aurons overdrive on the gui. if you use aurons overdrive PERFECKLY it can destroy the arms with one hit AND harm the body. switch in tidus and kimahri if you want to so that they can get a massive amount of ap after the battle. keep up with this pattern to beat the gui.
after you finaly beat the gui......itll come back. but it will only have half of its hp and the maester seymour guy will be in your party. seymour is very powerful and useful. when its his turn use his fira spell on the head,(yes the head is back also)to kill the guis head with one hit. after the head is dead again, use seymours 2nd level powers to destroy the arms to get to the body also. the arms will regenerate like before. when the arms are gone use seymours magic on the body. yuna will be in your party automaticly so use her magic heal the members if necessary.
i hope i helped and good luck with the boss and the game!*_*
