Shining Force 2: secret battle
This fight is against every boss in the game, minus the Kraken, plus two Prism Flowers. It's a challenging fight, but it's really fun. "Super" difficulty.
To get it, you'll have to wait at the ending screen that reads "Fin" for about four minutes (I think), until a dialog box that reads "And more" appears, then it starts. All of the characters you used in the party for the final battle, will be the characters you control.