The Steel Giant is a Soul of Rebirth boss in Final Fantasy II. It is found inside a chest holding the Ribbon. This enemy is similar to the Iron Giant of the main story but has extremely higher defenses.
Your best bet is to attack with Magic. Steel Giant has a weakness to lightning, so be sure to have a couple of characters with Lightning above levels 3 or 4. The others should be the healers of the party. Steel Giant can hit inflict a very high amount of damage. After the battle, this enemy might drop a random Genji item, Aegis Shield, or Excalibur.