FF2 Soul of Rebirth

To unlock the "Soul of Rebirth" dungeon, you need to clear Final Fantasy 2 to have the game ask you to save your "Clear Data". Once having done this, you should have access to the bonus dungeon Soul of Rebirth.

This dungeon contains other Final Fantasy monsters and bad guys such as Steel Giant and Cait Sith, as well as a large amount of treasures ready for you to find them.

I had very little problems going through this special dungeon, although there were a few close calls. If you are having problems, I would assume you just need to have more patients to level up your party members a little more. It is also a good idea to change strategies or try new tactics when fighting bosses if you are having problems with them. Usually the problem is just that your strategy needs to be changed, as the same method of killing one boss doesn't always work on another boss.