<Edited nerd rage ramble>
...Well, you can fight a superboss by doing all the Cie'th stone missions. The final mission, 64, has one of the toughest enemies in the game.
Also, after doing the "stone circle" missions, you can fight a Long Gui which is extremely difficult to kill.
There is a few, there is Attacus the Soulless, one of the Cie'th stones in Titan's trial. That mission 64 boss is hard too.