The best way is to make sure the overdrive gauge on all of your aeons are full. Then just attack, attack, attack. When you run out, start attacking that little alien-looking thing. It will drain Seymore. I'd make sure I have Tidus out there to speed things up with Hastega. Oh, and make sure to heal your people with holly waters to keep them from dying from that zombie effect stuff Seymore uses!!

Hope that's enough. . .