This isn't just a rock, is it?
It is a powerful creature that uses three different attacks: a Breath attack that is Fire-based and will damage the entire party, Earthquake, that will also target the entire party and delay their next turns, and a regular physical attack.
The Adamantoise is one of the few fiends in the game that will drop empty four-slotted equipment. This equipment is ideal for customizing easily obtained attributes, such as the -proof resistances. When it comes to the choice of waiting for Nemesis to drop four-slot armor versus obtaining 100 Dark Matters, for example, the choice is less clear. Before being customized, empty equipment sells for little money, no matter how many slots it has, so Adamantoise is not a farming target in this sense.
An alternative to farming the Adamantoise for four-slot equipment is simply to buy it 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 Grand Maester Yo Mika.
Adamantoise can be more easily defeated using Armor Break or piercing weapons. Using Ifrit against Adamantoise is quite effective. Once Ifrit is summoned, it heavily favors using its fire-element Breath attack, which will heal. If it does utilize 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 itself. 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 on average 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 grant no AP when defeated.
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 to earn AP off of in the Sea of Sorrow as you can safely switch characters in during the turns it is asleep. Once it awakens Ifrit should be summoned to easily 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.