The Dark ing is weak to the spell Cure, so just use Cure on him 2-5 times while using Phoebe as both your healer and to cast White
On any battlefield that rewards Experience points, keep fighting encounters until you have just one battle left.
Then go anywhere else and continue fighting enemies until the amount of experience you require to gain the next level is about the same as the experience awarded per fight back at the earlier battlefield.
Return and complete the battlefield, which should cause you to gain a level from the the experience awarded from enemies, then another level!
Curiously, you can use this game to check the version number of the chips used in your SNES. To do this, simply press Start + Select at the same time on the menu. The text may appear glitchy after performing this: simply exit and reenter any menu to fix this.
In Doom Castle, you fight a boss called Skullrus Rex. He is another version of Flamerus Rex from the start of the game.
Swipe him with the Dragon Claw and he falls, in one hit.