It has high Power, and will always attack the character that has the lowest HP. It is armored, but has low Magic Defense. They also appear in unison with the Gemini enemy in the Omega Ruins. They are excellent sources of AP, yielding 1200 if overkilled.
Iron Giants will drop armor with SOS Regen (which sell for a high price) and/or HP +5% or 10%. They will also drop weapons with Strength or Magic bonuses of +5% or +10%.