Nioh and Nioh 2 (including all DLCs) are also bundled in the Nioh Collection on PS5.
William will fight Marume Nagayoshi in The Way of the Warrior: Adept and The Way of the Warrior: Veteran which are both Master Missions. The battle is a sparring match, which could be relatively easy if you know how to use your weapons well.
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Marume Nagayoshi Boss Guide
In both missions, use a spear in mid stance as it has longer reach, making the boss unable to attack you since he only uses a katana. Wait until he is finished with his attacks and then strike him with multiple mid range blows.
Once his Ki is depleted, he will fall to the ground, this enables William to land a strong attack for massive damage.