To easily gain experience, find an area with strong battles and defeat all enemies except for one. Then, put everyone in the party except the characters you want to level up on the sidelines. Kill the remaining enemy. All characters in battle will split the experience point total, while all characters on the sidelines will receive 10% of their full experience regardless of the number of characters on the sidelines. So you actually gain more experience by keeping most of the fighters on the sidelines.