As long as the extremely hard super boss is Ultima Weapon I'll be happy.

Anyway, the boss should have a move that make the characters completely useless. Like, drop their max HP to 100 and all stats but agility to 1-4. This force you to change party members during the battle (like in FFX). And use the other party members until the effect wears off.

Another move should be that the boss start to controll one of your party members and use that characters best moves over and over again. You will have to kill that party member to get him/her back to normal.

Also, the boss should have a combo move that first takes away all HP but 1... and then, the next time the boss attacks it uses a move that takes away 50% of all characters MAX HP. This means that you'll have to heal atleast 50% before the boss next move.

And finally, the boss should have a move that makes itself hasted and invinsible for 4-5 turns.

Oh, and FINALLY (this time I mean it), the boss should have a move that cast dispel on all partymembers, and then something similar to Bad Breath and then take away 50% of the characters current HP. All in 1 move.