What would the cr be on.

[shilsen]

And what is the ninja rule, Shilsen?

Jdvn1 and Notmousse are close, but it's specifically that the power of an opposing ninja is inversely proportional to the number of ninja remaining. It's based on the fairly cliched bit in martial art movies where the heroes will be attacked by a large number of assailants, and the more of them they kill, the tougher the remainder seem to be, till the last one or two will put up a really good fight.