It's best not to use any elemental magic against Tiamat, as she'll counter with Maelstorm, bringing all your party members down to single digit health. This can be fatal if she follows up with Thermal Ray, which affects the whole party as well.
She can use Venomous Gas to poison the whole party and deal moderate damage. The damage from poison is not really great, so this should not be an issue, unless the party falls prey to Maelstorm or weakened by Thermal Rays. However, even if the party heals, she can recast Venomous Gas, so healing poison is unwise. One should focus on attacking.
After being weakened, Tiamat can cast the powerful Thunderbolt, which can deal massive damage to a single character, likely knocking them out in one hit. She's also capable of casting Lightning, inflicting thousands of damage on the party. If possible, equipment that resists Thunder should be used. Shell also helps greatly at this point.