Here's one strategy:
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Another is to Grand Summon Yojimbo and pay him enough to one-shot Seymour. I've seen reports of 30K triggering it, but Yojimbo is pretty random x.x
The Aeon strategy is to go into the fight with Yuna's Overdrive maxed and at least 4 Aeon's Overdrives maxed. Summon an aeon, use it's overdrive, Seymour banishes it, then repeat with the rest of your Aeons, Grand Summoning the 5th. Bahamut breaks the Damage Limit, so save him for last. This strat should finish him, though it does feel pretty cheesy.
General tips for Seymour Flux:
He is susceptible to Bio, Poison, and Silence. Use Wakka's Silence Buster to prevent him from casting Dispel, Reflect, Protect, and Flare. Hastega is a big help as well. If he is not dead before he uses Total Annilihation, make sure to either cast Shell on the party, use Kimahri's Mighty Guard, or one of Rikku's "Mighty" mixes, then defend.
Link for general info about Seymour Flux and Mortiorchis:
http://finalfantasy.wikia.com/wiki/Seymour_Flux