Jeff points us to the law of Conservation of Ninjitsu, which is awesome:

In any martial arts fight, there is only a finite amount of ninjutsu available to each side in a given encounter. As a result, one ninja is a deadly threat, but an army of them are cannon fodder.

This can apply to Elite Adversaries other than ninjas; vampires, for example, are particularly susceptible to Conservation Of Ninjitsu, as are werewolves, Special Forces commandos, and Super Powered Robot Meter Maids. Zombies seem to be exempt from this; they’re mainly dangerous because they travel in packs.