Throughout the years, I always pushed buying the Flock of Swallows scroll from Muramasa to the late chapters of the game, believing it wasn't a very useful technique. Today, on a Master Ninja no Ninpo run, I had a lot of spare yellow essence (from not buying the different Ninpos), so I bought the FoS scroll earlier and tried it against Spirit Doku. It works WONDERFULLY, specially when he does his dash attack. Literally, four or five good connects of the FoS against Doku and he was down, on the first try. Not only is this relevant because Spirit Doku is one of the hardest bosses of the game, but also because that fight can last a good 3 minutes on harder difficulties, more so if one dies, and today I beat him in about 40 seconds. In a Speedrun, this technique would be extremely useful and faster than any other way of fighting him. FoS scroll is a must buy from now on.