I think the last boss should offer a moderate challenge as Kefka did. Granted I beat him with only three characters to see the minmal ending before. I used the genji offering of the atma weapons to beat him though.

I think the last battle should look cool and present a feeling of awe, as kefka does. Getting one shotted by some badass boss when you are so close to the end of the game, would not be fun. Star Ocean: Second Story was like that for me, but I was only level 70.

With the proper leveling and over preperation, I think the boss should be fun, rather than challenging.

bipper