It's a bit tricky, but I think this works the best.

-Start off by casting hastega on your members.
-Have Lulu cast Bio on Seymour. This will do 1500 damage per turn.
-Immediately after Seymour hits you with the Lance of Atrophy, use a holy water on that person. You should have a turn to use the holy water if you have hastega casted.
-At some point, Seymour will cast dispel on your party. This is an indication Mortiorchis is about to use Cross Cleave. Make sure your characters with the greatest amount of HP are in, and try to cast protect on as many of your members as possible before he hits you with cross cleave.
-After you take off 35000 of Seymour's HP, Mortiorchis will go into auto attack mode. Start casting shell on everybody, as this is pretty much the only way you will survive Mortiorchis' Total Annihilation attack.
-As for attacking Seymour, just use Auron to wear him down and have Lulu cast Waterga, Blizzaga, Firaga, or Thundaga on Mortiorchis. A well built Kimahri would really help for this battle, as his attack power will increase if he talks to Seymour. Also, using Mighty Guard isn't a bad idea either.

Good Luck