Fight in the grassland area around Doma castle. Make a party and have someone cast Vanish on Gogo or Umaro (depending on who you are trying to level up and they should have the Exp. Egg equipped) then kill the rest of the party until they are the only ones standing and go at it. If an enemy manages to knock them out of vanish, just revive one party member with Vanish/ Phantom summon, put the effect back on, and then have them off themselves so Umaro/Gogo get maximum XP. I find this method works pretty well.