If you're on Mt Gagazet, then you might have to train up a bit. There's a save point just before you enter the top of the mountain, so go down and find some random battles and train up a bit. It's really hard, but it helps if you:
a) Get all your aeons to overdrive. On random battles, get one out and use that to boost its overdrive gauge, then dismiss it.
b) Obviously, save a lot.
c) When you feel ready, go face Seymour. Hit him with all your aeons overdrives, and it's cool if you have Yuna at full as well, then you can use an overdrive twice.
That's the way I did it, but I also went back to that save point and did a few hours of training them all up at that save point before I realised that theory might work. Good luck!