Level

An Adept's Level is determined by how many experience points that Adept has. The higher your level, the higher all your other Statistics will be. In the Golden Sun series the highest level you can raise your Adepts to is Lv.99.

Increasing Levels
The only way to increase your characters' level is by engaging in battles. Typically bosses yield more experience points than others, but since bosses are much more powerful than normal monsters and are generally only encountered once, fighting bosses is not a feasible way to train. The most effective method of training in Golden Sun: The Lost Age is killing the monsters of the Phoenix monster line, which are famous for dishing out large amounts of experience points.

Also, felling any monster with a Djinni that is of the element that monster has the lowest resistance to will yield increased experience points. For example, felling a Wonder Bird with a Mercury Djinni will cause you to gain 11208 experience points, rather than the usual 8622.