Appears in: Tokugawa Cave
Drops the best armor in the game (Peach/Warrior set).
Use a Blockbuster Scroll to destroy the walls allowing you to fight Masamune without also having to worry about surrounding enemies.
Alternatively, use a mass area damaging scroll to get rid of the mages (wait for Masamune's magic barrier spell to break otherwise it'll hit you too).
Alternatively alternatively use a magic barrier spell yourself, and this will cause Masamune's barrier spell to break.
Aternatively^3, use magnet to pull in the mages (and treasure chests) as the fight will be easier with them defeated.