Adamantoise is an enemy and boss in Final Fantasy IV: The After Years.
The Adamantoise is reasonably powerful, but is weak to Ice. A simple Antarctic Wind or Arctic Wind will defeat it. If the player has none, the Adamantoise can still be defeated easily enough with normal attacks.
As the final boss of the chapter, the Adamantoise is much stronger than in Edward's Tale. It is immune to damage at first, and the player must wait until Ursula unlocks her Tenketsu ability, exposing the Adamantoise's weak point. The player can now use the Five Star Crimson Palm Band with Yang by combining Tenketsu with Yang's Attack command. As long as the player keeps their HP up they should not have any difficulty, as the Adamantoise only uses physical blows.
Another strategy is to put Yang and Ursula in the back row, and have Yang Cover Counter and Focus while Ursula constantly heals him.
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.