How will I get to Iron Giant in Final Fantasy III?

Question

I finished the game but theres no more fun when theres no more challenging enemies.

Answer 1

If you are talking about the hidden boss in game, you have to unlock the mail by the 4 old guys telling you about a cave and King Artus mentioning a legend. Then from the floating island go east and you will see a shadow in the water, use the nautilus to enter the cave.
I advice you that the Iron Giant has the ability to hit you 4 times and can cast high level spells. I haven't beaten it yet since I don't want to grind to level 99 with lv99 onion knights. A good indicator that you are good is if you can kill red dragons w/o a problem and in less than 10 minutes.