The Masamune Blade can be found on floors 31-40 being used by random screamers
Gaijins can also make this weapon by finding a blueprint and turning it in.
An easy way to get this weapon without much hassle, once you unlock floor 31 all the boxes on that floor have a chance of giving you the blueprint for this weapon, simply open all the boxes, run to the elevator, then rinse and repeat.
|UNIQUE MASTERY RANKS|
|Weapon Mastery Level Click here for the universal Mastery Ranks|