Haven't played with a Ninja yet but... as a counter to Apache's well made observation of the Ninja's nigh-invulnerability, would it be possible to play a modified golden BB rule (The Ogre Book, 2nd Ed., p.101 &
here ) to allow the stealth feature to be disabled? Same idea as a critical hit but instead of destroying the Ninja outright, the -1 to attack rolls is removed. You could probably also use this against regular Ogres to give a +1 to future rolls. Just a thought...