there are four characters you'll want to be using for this fight, Tidus, Yuna, Lulu and Auron. Make sure Lulu knows Bio and all Her -daga spells, make sure Yuna knows Dispel, Protect and Shell. Make sure Tidus knows Hastega, which he better by this point in the game. Before the fight you should get all four of those characters (with the exception of Lulu) to their Overdrives. Also summon Bahamut, Shiva, Ifrit and Ixion and get them to their Overdrives too. The last preparation to make isn't a necessity, but protection from the Zombie status will also make your life much easier, as will a hefty supply of Holy Water, but as long as you have a lot of Phoenix Downs or Life magic you won't have to have it. Begin the battle and immediately have Tidus use Hastega, Auron use his Overdrive Shooting Star and Lulu cast Bio on Seymour. Bio will be probably the biggest source of damage you use for this boss and it's so simple, be absolutely sure you use this! Now switch Lulu for Yuna and have Yuna summon (not Grand Summon) one of those four Aeons every turn. Each of the Aeon's Overdrives should deal 9999 to both Seymour and his Mortiorchis. The Mortiorchis will drain Seymour and your attack will have dealt 13,999 damage. Have Auron use his powerful normal attack for awhile, soon enough Seymour will inevitably cast Protect on himself and Reflect soon after that. Keep this pattern going for awhile, then make sure you use Aurons most powerful normal attack, make sure before he uses Mortiorchis you have cast shell this will lower some of the damage taken that would have killed you off, but vefore he will use flare on himself as he has reflect on use dispel on him to reverse this effect then bring out shiva and use her diamond dust attack which will do 9999 damage and keep it up he will be finished