Have the Judgment Staff in your inventory so that Flare can be easily cast. Have all but one character dead in your party, and to be safe have that character be at least level 50. Then, go to the second floor of the Chaos Shrine. Fight Green Dragons and Black Knights that reside there. When in battle, use the Judgment Staff to cast Flare, which will instantaneously kill the foes (although a second round of Flare casting may be needed). The experience points (EXP) dropped by the Green Dragons and Black Knights is staggering and will allow you to quickly level up your character, one at a time.