This is the easiest way,

Put all aeons in overdrive in previous battles, equip Yuna with a sensor weapon. Once Seymour's HP gets to 39,996. Summon over and over using overdrives, He won't even get to use Total Annhilation.

As for surviving Cross Cleave, Seymour always casts Dispel before using Cross Cleave. If Yuna or Rikku (use), (and possibly Kimhari if he used Yuna's path) gets a turn before he does it, Cast Protect on any character who's HP is between 1270-2580 or use an item that casts protect, use a Mega Pheonix to revive lost characters if more than one is dead.

When you start, cast Bio with Lulu and use Hastega, especially after being dispeled