Fighting
Tiamat is like fighting all four types of
Dragons at once. She alternates between their
Special Attacks, all of which can be devastating to those not defended against them. The
Dragon Mail and
Aegis Shield will cover all four
Elements, as will a
Ribbon, but if you have anyone left unprotected,
NulAll is the quickest way to change that. You could also lead with
NulBolt to counter
Thunderbolt, but there is no Nul
Spell for
Poison specifically, so
Toxin will still hurt.
Tiamat's Magic Defense is as high as it gets, and she sports resistance to all three basic
Elements (as well as
Earth), so most offensive magic is useless. The exception is
Poison-elemental
Spells.
Break is a bit more likely to work, if you have a
Black Wizard to cast it, but
Poison (perhaps cast with the
Venom Blade) is nearly as good. If you have a character with nothing better to do, it's not a bad idea to try for the one-shot kill. Fighting more traditionally is trickier, as
Tiamat has a whopping 80 Defense.
Steel and the
Giant's Gloves will help get you through that, and
Haste is always a good idea. She also has four attacks that each do significant damage, but you're not likely to succeed with
Slow so stick to defensive
Spells like
Shld2, and perhaps a use of the
White Robe. If you have elemental protection,
Tiamat's main form of attack is blunted, and you should win the day with a sustained assault.