This Seeking Tech guide will show you the fastest way to finding the Iron Giant in Final Fantasy II Pixel Remaster.
You can only fight against this enemy on the fifth, sixth and seventh floor of Pandaemonium, which is the final dungeon in the game.
The best spot is apparently on the seventh floor as there is an area with a line of pillars on the right side of the map. If you run below these pillars, then you may have a better chance of eventually encountering the Iron Giant.
In terms of the actual fight, the Iron Giant is weak against Ice and Lighting magic spelss. When defeated, it can drop the Genji Helm, Genji Gloves, Genji Armor, Excalibur or Aegis Shield.