How Do I Get The Last Crystal to save rosa?

Before you go to the cave you must speak with edward and get the twin harp, this is an absolute MUST because if you don't then you WON'T get the crystal, any way after you have gotten the twin harp head north to the chocobo forest get a black chocobo, then head east you should find a cave surround by sea,forest, and moutain, land in the forest and enter the cave, when you enter someone should say something along the lines of "you can't use metal things," know if you want to train remove anything metalic (eg sword, armor, sheild, rings, etc), but if you don't then just run from all the fights, eventually you should come to the dark witch, don't try winning the first time because it is impossible, know once you are dead edward will play his harp and the song will be transmitted through the twin harp know equip all your normal items because you can fight normally again abnd know just use your normal tactics and you eventually have th crystal.