Here's a great way to level-up near the end of the game if your party is in dire need of experience (or gold). Head to the Ancients' Maze, which is located outside of the Crystal Tower. Here, you will find an enemy called a Greater Demon. This creature attacks twice a turn and has around 10,000 HP.

These creatures can also summon monsters called Iron Claws (making this technique similar to the one employed in Final Fantasy IV near the Summoner Village). If you kill the summoned enemies but not the summoner itself, you'll allow it to keep summoning more enemies. This battle can theoretically last hours (as long as the Greater Demon isn't killed) and can net you a ridiculous amount of experience and gold.