If you have some Doors of Tommorows, just mix them with Wakka there and Tidus in OD. Then, use Slice and Dice(not sure if that's the real name, haven't played in a while), and it'll do some heafty damage. Then, have wakka continue to attack, switch out Rikku with Yuna, summon OverDriven Bahamut, use Megaflare, have him banished, then keep on attacking.