ummm... does anyone know that if you junction 100 firaga and 100 thundaga to your elemental defense, when you fight Tiamat, since all he can do is use Dark Flare, the damage will be absorbed as Health (like if you use any fire magic on a Bomb). So just go and draw from a few Jelleyes until all your party have 100 firaga and thundaga and you have a party which Tiamat can't hurt at all!

(PS - your party must have at least 1 of the following GFs each in order to have at least Elem-Defx2: Quetzalcoatl, Shiva, Ifrit, Leviathan, Pandemona, Alexander, Doomtrain.)