Ultimecia's Castle Problem

[Noj_R]

His only attack is fire-elemental, so just junction some fire elem-def properly (over 100) and Tiamat will only be able to heal you; then do as you will.

He's actually the easiest of all the boses in Ultimicia's castle... if you junction.

Exactly, junction 100 fire to your elem-def and you are invincible. All you need to do now is defeat him as you see fit.