Immediately after the scene where the strange voice tells your characters to swallow the light spheres (after fighting the land shark), there is a chance to gain at least three or four levels in both class rank and character level. You have the choice to escape using the Mechat machine, but do not. Instead, fight the horde of robot enemies. Note: This may take awhile, but the experience gained can really help your characters for what later in the game.
The Underground River area on Disc 2 is a good location to level up your characters fairly easily. Go to the end of the area where you can find lots of the Kelelon things, then just keep battling them repeatedly as they will keep respawning. Note: Try using RT to surround as many monsters in your radius as possible. The Underground River is also has lots of other respawning monsters.