This isn't just a rock, is it?
Adamantoise is a powerful creature that uses three attacks: a Fire-based Breath attack that hits the entire party, Earthquake that also targets the entire party and delays their next turns, and a regular physical attack.
The Adamantoise is one of the few fiends to drop empty four-slotted equipment, ideal for customizing. An alternative is simply to buy four-slotted empty equipment from Wantz at the south entrance to Macalania Woods, if the party spoke with him at Mt. Gagazet (otherwise he will disappear). He moves to Macalania after Yuna confronts Yo Mika.
Adamantoise can be more easily defeated using Armor Break or piercing weapons. Summoning Ifrit against Adamantoise is effective, as it favors using its fire-element Breath attack. If Adamantoise does use Earthquake, Ifrit will face the damage from it, but is immune to its Delay property and can heal easily by casting a Fire spell on himself. Using the Abilities menu to upgrade aeons with buffs like Haste and Protect can extend their usefulness, while debuffs against enemies, such as Armor Break, can often just as easily be done by the party.
Parties with Stonestrike weapons will make shorter work of the Adamantoise if their battles with it would normally last longer than five attacks (it has 80% resistance to Petrification), but petrified enemies do not grant AP. A good strategy is to cast Haste or Hastega, then follow up with Armor Break and hit the Adamantoise with Quick Hits. The optimal party for this would be Tidus, Auron, and Wakka.
The player can also take advantage of Triple Foul, which will put the Adamantoise to sleep for three rounds. Combining this with the previously mentioned Ifrit strategy makes Adamantoise one of the best enemies for AP farming in the Sea of Sorrow as the player can safely switch characters in as it sleeps. Once it awakens, Ifrit should be summoned to kill it.
Adamantoise is a portmanteau of "adamantite" and "tortoise".
Adamantite and similar words including Adamant and Adamantine are used to refer to any especially hard substance, whether composed of diamond, some other gemstone, or some type of metal.