I've defeat Tiamat 145 times, so... I know her very well.
The best way is having Zidane with Ultima Weapon, using Soul Blade, so that Tiamat falls asleep, then with Dagger use Bahamut, Steiner with Climazzard and Vivi with Flare, do this during some time and Tiamat will rarely attack you. Gotcha?

Don't forget... In the map where Tiamat is there are the Rune Claws, the best weapons to Amarant.