Originally Posted by Gamefaqs
While this battle isn't very hard, it turns out being tricky. Your best
bet is probably striking physically with at least 2 characters, have a
White Mage to ALWAYS heal the injured party members, and you can add a
Black/Red Mage if you want, or trade him for another physical attack. In
case you actually use a Mage to case offensive magic, try to go (first)
for the more powerful spells and then for the weaker ones, but stay away
from fire-based spells, as they seem not very good to use in this battle.
As you may notice during the battle, the most important features about
this boss is that it can turn your characters to stone, or simply make
them unable to cast spells. If any of those 2 effects are induced in your
characters, be sure to use the proper item (or even a magical spell) to
remove such effect, as soon possible.
By avoiding these 2 effects, healing with the White Mage and attacking
with everyone else, your enemy will probably go down in no time. When I
fought this battle, my weakest character was at level 14, and I strongly
suggest you to do that very same thing, in order to have an (even) easier
time fighting this mythological foe.