Once you open up the area "Cave of Ordeals". Go through it until you reach the 3rd area. Here you will find 3x3 rows of enemies and the field has a "Exp 50%" Geo panel on it. If you can use a 3x3 attack to kill all the enemies in this stage in one swoop, your character will level up insane amounts. You can take a level 1 character to level 200 if they have a high enough attack. Also be warned that the enemies in this stage have high defense, either bring out other members to weaken the monsters up or you can have one warrior bravehearted by a team of mages.