Kacho Goes Down Without a Fight
The Butterfly Collector starts off as an inoccent game of finding origami butterflies but soon devolves into an investigation of a child trafficking ring. 
Kacho is the final target in the story arc. Ordinarily, approaching her triggers a short cutscene followed by another annoying boss battle. This is just a quick video to show how to avoid combat with an easy air assassination.
I enter Nishinomaru Garden through the hole in the wall on the west side. From there you can proceed ahead to your left directly to the assassination point. In the video I first go to the open gate and identify the target from ground level. Then I backtrack to the hole in the wall and climb onto the roof above Kacho. As long as you stay high on the building it won't trigger the cutscene. I was somewhat surprised to find that air assassination is possible. Every other assassination style I tried lead to the boss battle.