Kisho ninjas are army units that can be deployed in Shogun 2. At first glance the unit seems ideal for surprise attacks (they can hide in plain view and deal damage quickly). However they are small in terms of army count and fragile, which makes it hard for them to survive battles without significant losses. What are some tactics which could be employed to field kisho ninjas cost effectively?
Kisho Ninjas stealth, bombs, and (relatively) high strength can make them good distractions with their small sizes.
Kisho Ninjas are best used to disrupt enemy flanks, assassinate your enemy's generals, remove ranged archers or other ranged forces, or sneak into castles.
Option A: You can hide them, use your main force to attack, and then use them to throw bombs at the occupied enemy forces. (Your Ninjas should be out of harms way, thus prolonging their use.)
Option B: Sneak into undefended (or alternatively lightly, defended) castles.
Option C: Lure units away with the Ninjas from areas you want to occupy (say Tenchu circles) and go in for the kill while enemies are distracted. (Alternatively, a more expendable force can be used to lure them away, and use the ninjas to provide support or kill off their pursuers).
Option D: Banzai!!
Experience from playing Shogun 2 long enough.
Edit: As an alternative, you can always use Kisho Ninjas to defend castles, using grenades to take out climbing enemies. This way, your Ninjas (if not being rained upon by enemy arrows) will be out of harms way while quickly demoralizing the enemy.