Don't go up there for a while. Grind out some levels in areas before him and then roll back when you're extra buff. Its been forever since I played it, but thats what I ended up doing.

Heheheh, oh I can really relate to this. :P It's just another one of those classic FF bosses where you have to level up the hell out of your people before you fight him. It's enough to put you off the game entirely.

First off, everytime you see the ''Lance of Atrophy'' attack, ready yourself some Holy Water, Esuna or Remedy, and use it on the character that his attack will hit, before Seymour can then cast Full Life on him. This is a pain, but avoiding this is key to making it through the first half of the battle. To help this tedious overside use Tidus to cast Hastega on your party. (Also, Zombie Ward and all, consider this, although this takes forever.) When you have a chance, use Lulu to mess up Mortiorchis so it absorbs HP away from Seymour. The key is to keep everyone fully healed as best you can, because Cross Cleave is the least of your worries. When his HP starts getting low, Mortiorchis starts charging for an attack called Total Annihilation, which will probably ruin your entire party but, either Khimari or Auron should have over 4000 HP to survive the attack and revive the others. Since it charges for this attack though, I suggest bringing in Yuna and casting Shell on your party to greatly reduce the damage. Or, if you've learned it, use Khimari's Big Gard spell to reduce the damage by half. To damage the boss, use Aeon Overderive, if you've built them up, they should do maximum damage, and Bahamut should break the 9999 mark, and Tidus should monitor Hastega, you need to be fast in this battle. Also, you can sacrifice your Aeons during Total Anhilation, so that they take the damage.

level the hell up, heal constantly, a strategy i found that works good is to use your aeons to shield you from his attack that damages your whole party, you can deal a shit load of damage and protect your party, also make sure to heal, and use protect and repel if you have them.

right before the battle make sure everyone has overdrives even the aeons.when you use yunas overdrive make sure she picks the strongest aeon so they will do the same overdrive 2x's in a row