The best way to earn experience depends on where you are in the game. During the begining you shouldn't worry to much about your level, just fight the monsters where you are and work more on hidden items and places that you need to come back to. When you get your sandcraft you can earn up to 1000 experience fighting monsters in the dunes (the sea of sand). This is good until you are about level 40-50, than the task gets slow and painfully boring. As soon as you are able start the Alien Invasion side quest (you find and fight alien towers to defeat the alien invasion) after defeating a couple towers Choas will apear as an enemy you randomly run into around the area of defeated towers. 1 choas gives you 10,000 experience, but they also will inflict almost every status adnormality on all your characters. It is best to fight them after you have gotten all the mediums then to use Gallows Force Ablility and put Status Lock on all your players, then use performance on all your charaters also. When you run into 3 choas at once use Virginia's Force Ability and the lucky cards to doudle everyone's experience. The lucky cards can be used once per character per battle so don't waste them trying to use more or on low experience monsters