Get enough Yin to create a Keep. Get four Ninjas. Get four Ronins who have Blood Bond battle gear. Blood Bond each Ronin to a Ninja, then send the Ninjas up the four Watchtowers. Send the Blood Bonded Ronins to the enemy base and watch them die. Notice that the Ninjas are still in the watchtowers and they are regenerating. Click on your keep and summon more Ninjas. Note: A Geisha, allied or your own, must not be near the watchtowers.