I couldn't finish 'em. Looks like this's gonna get complicated.
Tiamat tends to be a more straightforward fight than the other bosses in Pandaemonium. It has powerful stats, but does not do anything special beyond casting high-level spells of each major element (Blaze XVI, Icestorm XVI, Thunderbolt XVI, and Poison Mist XVI) and landing heavy physical blows. Tiamat resists most anything, so the fight can take a while. Tiamat will absorb any elemental spells, but Ultima or Flare will work.
It is advisable put up any magical barriers and cast Stun or Slow on Tiamat, then hammer at its HP with the party's strongest Blood Sword wielder. An alternate method of winning is to simply cast a level 8 or higher Teleport, Mini, and Toad.
Tiamat is a primordial goddess of the ocean in Mesopotamian religion. The game Dungeons & Dragons has Tiamat as a chromatic dragon, the Queen of the Evil Dragons, and as such the opposing deity to Bahamut, who is a platinum dragon and the King of Benevolent Dragons.