First, you have to head towards the Mountain in Kodama no Ki. Here is a direct link. Then, walk up the mountain by foot.
When you get to the open area, click on the Total War sign to get the HUD and the attachment.
Wear both, as well as your favorite sword (any sword will work), and wait for the enemies around the tatami mat. There are kinds of enemies; below are two Kodama no Ki ninja, but you might want to check into the house as well for other kinds of enemies :o)
Until the event ends, you have to kill as much enemies as you can. Good luck!
P.S.: They speak weird Japanese but... maybe they aren't human.