Treatment comes from Siren, or any GF who you teach the ability to with a Med Kit. You can get a Med Kit by refining 100 Dragon Fangs with GF Abl Med-RF. Might be other ways, but none that I can think of now.

On another point, does anyone know what element Tiamat's Dark Flare attack is? I went into the battle with 90% absorb all elements, and my characters absorbed the attack. I've been curious ever since, but I never remember to junction particular elements to Elemental Defense, so I haven't been able to figure it out yet.