At the beginning of the game's fourth disc, head to Urha after acquiring the Nautilus Boat following your party's reunion. Using Tolten, you can break the Urhan Seal in the Gongora Mansion's courtyard. This will give you access to a secret dungeon.

The enemies you encounter here are called Experimental Number 42, and you'll fight them continuously during your time here. These are tough foes -- they pack a punch and can petrify your party easily. If you're above level 35 or so, you should be in good shape to hold your own against these enemies. The key is to have some anti-petrify skills or gear equipped to combat their one deadly attack.

The SP these enemies give up when killed is staggering, and will allow you to level up quickly. There are no level caps here like there is elsewhere in the game, so that makes leveling up in this location over a long period of time up many, many levels a possibility if you have the patience to do so. Just remember to leave the dungeon every so often to heal and save so you don't lose all of that leveling progress!