Nioh and Nioh 2 (including all DLCs) are also bundled in the Nioh Collection on PS5.
Hattori Hanzo is encountered during Dojo Mission The Way of the Ninja: Adept and Veteran. Hattori Hanzo is a ninja, so his attacks are very swift and agile, he uses a Katana, Kusarigama and several Throwing Weapons that can deal short to mid range damage.
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Hattori Hanzo Boss Guide
Since Hattori moves very fast, you have to match up with his speed by equipping dual katanas in low stance. The low stance delivers faster attacks than any other stances while using the dual katanas. Dodge his attacks every once in a while and also avoid the shurikens that he will throw at you. Once he is open, proceed to strike him with consecutive blows. He has a very weak guard, so keep striking him once his Ki is low or depleted.
The same strategy can also be applied during your second battle against him.