The Dark Crystal Cheat
This works as far as I know only on FF2. But since FF2 is ultimately FF4, here it is. (ed: As far as I'm aware, it works for both versions!)
After you have defeated Calbrena and Golbez, you talk to Giott. After that DON'T MOVE and use warp (Rydia) this will put you in the crystal room and the crystal will still be there. Now all you have to do is merely walk into the sealed cave and Kain will take the crystal. You won't even have to fight the wall.
Submitted by: Adam
on the Sygnus Star network.