Have a saved game from SaGa Frontier 2 on the same memory card as the one used for Legend Of Mana. Enter the save screen, highlight the SaGa Frontier 2 file, then exit out of the save screen. Enter Bone Fortress and head toward the bridge where you fought the DeathBringer. There will be two skeletons at that location. Approach them and you will fight DeathBringer 2. After defeating it, you will get the strongest sword from SaGa Frontier 2.