If magic is used against Tiamat, she often counters with Maelstrom, bringing all party members down to single digit health. This can be fatal if she follows up with Thermal Rays, or any other ability that damages the entire party.
She often counters physical attacks with Venomous Gas to poison the whole party while dealing moderate damage. The damage from Poison should not be an issue, unless the party falls prey to Maelstrom or is weakened by Thermal Rays.
After being weakened, Tiamat can cast the powerful Thunderbolt, which deals damage based on her HP to a single character. When she starts using it, it can deal over 5000 damage, likely knocking the target out in one hit. She also uses Lightning, inflicting thousands of damage on the party.
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.