Well I searched but didn't find anything on this so I will post it. There is a chest in the Sea Caves that everytime it is opened or closed summons a monster to fight you. When I was doing it with a party of 6 it summoned a level 20 Rebel Spirit that was worth almost 100,000 exp. I have no clue whether it will summon the same Monster if you have fewer party members.My new party is only 4 characters and I think I will be using this chest for some exp and to proctice skills.