There are three items in this game that will allow you to gain levels at an amazing rate.

Learning Ring:Increases EXP by 30%.
Coin of Fortune:Increases EXP by 50%.
Book of Refining Gold:Turns each Monster into at least 10 Mystic Gems.

So, from this you can see that if you have a character equipped with 2 Learning Rings and a Coin of Fortune they will be getting roughly double the rate of experience that everyone else is. This will allow you to get a Character like Valkyrie to the high levels up fast.