Final Fantasy Mystic Quest is a Role-Playing game for beginners, so most of the game is a pushover. But when you get to the last boss, the game ramps up the difficulty significantly.
Spoilers continue below this line
The Dark King starts out pretty normal, until you do a bit of damage to him. Then he morphs into a kind of serious threat. Well, for this game anyway. But you have a secret weapon: your Cure spell.
Have your main character cast Cure on the Dark King
For supermassive damage! It even breaks the four-digit damage barrier. While you're doing that kind of damage, he won't last too long, so sit back and enjoy the ending.