Dawn of Souls: Final Fantasy II: Tiamat Boss
There seems to be several different versions Tiamat throughout the Final Fantasy series. This is only the second version of her after she was one of the Four Fiends of Final Fantasy I.
When she appears to kill the party for taking the might Genji Helm, she's much more tan compared to her original blue self, but she still keeps her arsenal of different magic.
Primary magic, such as Fire, Blizzard, and Thunder, only feed her more HP. Attack her instead with your secondary magic, such as Flare, Holy, and Ultima.
For comparison, here's Tiamat in action during the first match and endgame rematch against her in the GBA version of Final Fantasy I:
Final Fantasy I Advance Match:
http://www.youtube.com/watch?v=Hm4SlyinYvY
Final Fantasy I Advance Rematch:
http://www.youtube.com/watch?v=JcNtmP2yF4I