This really isn't bad at all! There's some balance issues, and I think it would have been better to introduce the Goblin before the Demon, but it's otherwise a neat idea!
What I would change is to make things that are more powerful than the Blue Wizard's bolt need to have a trade off, such as lower DPS (like the Orange Wizard's), or shorter range. The Demon has range greater than anyone else's, and since the mobs don't aggro upon taking damage, you can sit just ousdie of the Wizard's range and never take damage.