See I disagree - I think a solo-based one on one PvE battle would actually pretty fun and an interesting FFVII-reference that would be perfect for the Gold Saucer (although I disagree with Generic's idea about the Buster Sword, seeing as that is already in the game as the melee Limit Break ).

Continuing the parallels with the FFVII Battle Arena there should be handicaps inflicted on the player the more rounds they win but with greater rewards the longer they last (say, things like 'weapon break - reducing the equipped weapon's condition to 0%, Half HP - reducing your total HP by half it's current cap, Soul Crystal Broken - de-equipping your current Job (HW Jobs it would instead disable all the abilities above level 30), Item Level Down 50 - reduces your average item level by 50 points, level down 10, reduces your current class level by 10, etc etc.

By tweaking the difficulty it would also make it an interesting counterpoint to all those idiots who say the game "is too easy!" Say that when facing down something like Ultima Weapon solo in the Battle Arena with a broken weapon, your gear level and class level reduced by 50 and your Soul Crystal unequipped.