I'm not going to even pretend like I'm great at giving advice, I'm not, but I'll give one tip that someone else has allready mentioned, get yuna in overdrive before you go in *or* make sure Bahamut's overdrive bar is full, and summon him. You only have one turn before Seymour's banish's any aeons you might summon so immeadietly Mega Flare his ass. I took off about 25.000 hp of him that way,...or was it more? I cant remember, was a while ago. Do this towards the end of the battle, and uh,...try to concentrate on Seymour more then the mortibody. And when he zombie's someone, try and use holy water on then quick? Ah! been ages since I fought him. Personally I beat him first time I ever did it, however I think it was abit of a fluke.