The Adamantoise really isn't that hard of a boss, if you're prepared. If you are prepared, you'll be armed with the ICE spell.
A few blasts from this will kill the turtle quickly. Also, use the Notus(SouthWind, Blizzard, etc.) found
earlier in the cave.
If you didn't find the item, and are out of MP, or are just retarded and don't have ICE, you could be in
trouble. The Adamantoise has very high DEF, so most physical attacks are useless. Also, it has high ATK, so
beware. Use SAFE and BLINK, if you have them, and keep your HP as high as possible. Also, try to curse the
Adamantoise with the Ancient Sword to weaken it's attacks.