Stealth isn't so "stealthy" if you just pay attention. He should have a little number floater above him whenever he does damage; that would give it away. And even if there is no number counter, just paying attention to your health bar will give it away. Mostly, I think this is an intriguing idea, but won't hold up in the game; it's just a more subtle clone of Immortal creatures.
Ninja is overpowered, and so is Nightelf, but to a much lesser extent.