First of all: I have a hard time believing ninjas fought on battlefields, seeing as their very existence was something you didn't speak about(since Japs are bound by honour and whatnot and the very notion of "ninja" collides with this fact). Yet, warlords of feudal Japan would of course make use of their services more often than not but naturally again, not stuff you talked about.
So with all that said, remove the ninja hood. Ninjas can still pretend they're some generic japanese assassin and we get easier kills(well, if that's even possible seeing as they usually have extreme AGI builds).
Assassins can stay in the shadows and backstab in a different game, on the battlefield is where men belong.