How should I use Battlefield Ninja?
Wooded areas and fog are your best friends when using BFN. I set traps for the AI by using a unit or two to step outside a wooded area to draw attention, then ducking back in to waiting units of my BFN. Bridge crossings get almost too easy (especially in bad weather) by sending BFN across first (you must remove the fire-at-will option) and have them wait at the flanks of enemy units. Send a unit or two to the bridge which starts enemy movement, then have the BFN crash the flanks.

In some of my "just for kicks" campaign's, I would have an entire army of BFN's led by a Kensei general. THE best "bridge-buster" army bar none. Send all the BFN's across the bridge, and when they are in position, have the Kensei step forward onto the bridge and watch utter mayhem ensue. I don't know if cigars were present in Japan at that time, but if they were, your Kensei should have one ready, as smoking one will be just about all he'll have to do....