Dude! Darkshine Knight is easy! Did you class change the second time? He's weak against fire, use Fire Sabers if you have them.
And, get this, HE'S NOT THE LAST BOSS. There's another final dungeon, and two stronger bosses.
I think my hardest boss is the Dragon Emporer from Seiken Densetsu 3. I used Deadly Weapon to cut his maximum HP by 1/5. It did 11,000 damage. He beat my ass down. I killed him eventually, though.
No! Scratch that. The Black Rabite from Seiken Densetsu 3. If you cast a dark spell, he'll counter by summoning a LEVEL 99 ENEMY against you.