I agree that immunity is too much. Cold/lightning absorption, say 25%, would be a nice alternative.
Giving all incorporeal beings (ninjas and PMs) a separate % chance to avoid knockdown effects would be nice. I'm thinking less than 100% avoidance because they are not fully incorporeal.
Then address the PM problems listed here: http://forums.ddo.com/showthread.php?t=284786
With the deathward bug listed as top priority for bug fixes.