Normal: "A fiend that appears to be a machina, but is actually an empty suit of armor animated by pyreflies. Physical attacks are ineffective, but it is vulnerable to magic." Oversoul: "One angry suit of armor. Something must have happened when it oversouled, because now it focuses all of its attention on attacking White Mages." — Scan
Iron Giant is a fiend in Final Fantasy X-2. It is not a particularly difficult foe to defeat, though it does have high Attack power. Using Power Break will help remedy this, and for taking it down, any magic or physical attack should do. When it is Oversouled it has access to all four Level 2 elemental spells. Casting Nul-spells will help with dealing with this, or if the player has access to them, using Lunar Curtain, Shell, or Reflect will help in cutting down the damage. It'll also take its rage out on White Mages, so cast Protect on them if necessary.
In the Fiend Tale endings, a notable Iron Giant was actually a Pupu in an Iron Giant suit sent to observe the residents of Spira. After attempting to translate the strange "fiend", Shinra brought Iron Giant to the Thunder Plains where he witnessed the Pupu emerge from the Iron Giant suit and leave the planet with the data it gained. After witnessing this Fiend Tale Ending, the Empty Heart team appears in the Fiend Arena.