Let's Play 7th Fantasy (Cid) Episode 39: The Ice Cave
Finally it is time to go to Gariso's Castle via the Ice Cave (game calls it the Cave of Baron, I guess...). Inside are horrendously powerful enemies, one of which actually casts powerful FIRE magic (the King Moons).
A statue-like dragon called Monmo guards the entrance to the actual Castle itself. He has near-infinite physical defense just like the Swords you may encounter outside, but unlike the Swords he has a LOT more than 9 HP - as in over 1,000! The ONLY way to kill him is by using magic, and only Fire magic seems to do decent damage. This is why I stocked up on Bottles of Fire from Guanta - so that Cid can actually aid with the fight.
Be absolutely sure you use both the Star and Light Runes at the start of this battle and BOOST YOUR SPEED, because Monmo can Drain and Osmose away your HP and MP. Being able to dodge at least some of his Drain spells is crucial if you want to effectively wear down his HP.