Bad move! Here is some advice. It was what I gave someone else on this forum before.
Ok total anihalation is your problem than right. Ok as you probably know rikku has a considerably weak attack but amazing speed. Thus casting haste on her could allow her to take action up to 3 or 4 times a turn. You should also have a full overdrive bar for rikku that way when you see that he is about to use total anihalation use to items that either cause protect or shell. Thise will cast super mighty guard on all party members. Have auron and all the other party members who have the highest heal. Have them all defend while they have super mighty guard still in affect. (of couse keep rikku but still defend with her) You will most likely have at least 2 members still alive if everything goes according to plan. Also be sure to cast super might guard AFTER he uses dispel. The easiest way to defeat him is to have all of yunas aeons overdrive bar full and use them. That makes the battle SUPER easy. Hope I helped. Tell me if you are still in need of help. Thank You for posting.
Lets live together, I don't know how long it will last, but it doesn't have to be for show, together, till death do us part. - Kenshin "Shinta" Himura - Rurouni Kenshin