Note: You must constantly save your game in order to do this trick. At any time when you encounter a Boss, immediately save the game to a new file after defeating them. Press Start and load the game file that was saved immediately before you fought the Boss. After the game has loaded, press Start and go to "Characters". Import the character from the file that was saved when you defeated the Boss. You will now have a character with more experience, but will be able to fight the Boss again which results in lots of experience. Keep doing this every time you defeat the Boss to gain levels faster.

After you are in to the game a short amount of time, talk to everyone in the tavern for easy experience. Also check periodically; as you complete more of the first part, there is more experience to be gained in the tavern.