Here's how I did it yesterday:
Go back to Oaka's brother and buy buy that "potion" that cures zombie (I don't know how this item is named in english - in german it's "Weihwasser")
Then, let Rikku add Zombie-Protection to your armors.
And of course, have all limit breaks full.
Then goto Seymour, and start shooting at him with all your limits - except Yuna.
Wakka is able to do an attack that makes Seymour "mute" (once again - I don't know how it's named in english). If you attack him with that attack - he won't be able to cast anything - this can help you sometimes...
Then, when you finally got him to ~10000 HP, you take Yuna, and then you make her summon Bahamut in Limit Mode. This should cost seymour over 10K HP...
I needed 6 or 7 tries until In was able to beat him.
If you are still not able to beat him - then you need to train. The area in the mountains, directly before the battle with Seymour is perfect to "tune" your characters a bit...
Good Luck!
- Alhexx