I used Yuna Tidus and Auron. During the Anima when Anima is about to overdrive call out a summon to protect yourself thn once Anima is beaten with Seymore I had Yuna cast all four null magics on the party (I started from the once in which he was about to cast emembering his last spell) so it's important you learn the sequence of his magic attacks (if you have the help option on when you do a command on seymor it will tell you it I think it's lightening ice fire water but not so sure sorry) also remember what last magic attack his did or you'll be sorry and then since he has shield I know magic wouldn't be that strong so I had Tidus and Auron just simply attack him. after that i didn't take to long at all.