Can't stun him, eh? As some others posted, my parties had no problem with Flood. However, this means you need a DRK in your party. (Why not use DRK as tank? People say PLD is best tank but if you think about it, the "tank" will only ever have one mandragora on him, and it hardly does enough damage to require a PLD... And now you have stun) Stun works, Weapon Skills work (though you probably won't have TP by now) but what I found best was Silence.
If you do like I do, party will have BLM, WHM, and RDM in it. BLM is great because they can nuke OR heal, depending on how the battle is going. WHM is your high-quality healing, RDM is great for Bard healing because he virtually can't run out of MP (Convert ^^) We had me (BLM) start with Elem Seal + Silence, and the WHM did the same when my Silence wore off. I found that, even with an undercapped Enfeebling skill but with high enough INT, I was able to silence this guy midway through Flood with a non elemental-sealed Silence.