When inside Kresselack's Tomb, past the first big door, will be a Skeletal Mage named Mytos. If you are polite with him, and opt for a nonviolent solution, you will get extra experience points before the fight.

To level up a character easily, have only one character. Go through the first quests at the starting town. Then, save the character as an import. Start a new game and load the import. Do the same quests as if you were using a new character. It does more difficult to level up, but it is possible to get your character to level 22 before going to the next town.