First off, have Kimarhi use Stone Breath to get rid of the Guado guardians. Then have Auron use Magic Break, Yuna use her Nul spells, and have Rikku handinging out Hi-Potions to anyone who gets injured. When he summons Anima, call the aeon marked ??????. If you manage to kill it and Seymour with an aeons' Overdrive you will be rewarded with a lot more AP and items than usual.