Four hundred years prior to the modern time of Final Fantasy, she tossed the Lufenian civilization into ruin, with only a village, a single airship, and her personal haunt left standing. With the Wind Crystal under her control, the wind itself has died down. As the first to awaken, she is the most powerful of the Four Fiends, and uses several powerful attacks. Tiamat also appears on the fourth basement of the Chaos Shrine 2000 years in the past, where she is slightly stronger.
Since Tiamat is resistant to all elemental spells, the most effective magic attacks to use against her are Flare and Break, which she doesn't have a resistance to. She generally uses either a strong physical attack to hit for around 250 damage, or two area attacks. The first is Thunderbolt, a Lightning-based attack which can be mended by Healara. It is also a good idea to cast NulShock.
Tiamat's other attack is Poison Gas, which, while not as strong as Thunderbolt, may inflict the Poison status. In the event a party member is poisoned, it is probably a better idea for your White Wizard to keep using Healara to cure the physical damage instead of wasting turns using Poisona.
In the NES version, using the spell BANE on Tiamat has an extremely low chance of instantly killing her. In the Dawn of Souls and future remakes, if the player uses a Cockatrice Claw on Tiamat, she will die instantly. She is also susceptible to Poison, so Scourge or Venom Blade will also affect her, dealing an instant kill.
Tiamat must be fought again in the Chaos Shrine of the past, along with the other Fiends, so she cannot summon Garland to the past time to form the loop that created Chaos. As well as the other Fiends, she has lost her weaknesses and is now more powerful, making her a tough opponent to deal with. Using Haste, Saber and Temper to enhance your physical attackers' strength will make short work of Tiamat.