Nioh and Nioh 2 (including all DLCs) are also bundled in the Nioh Collection on PS5.
He seems to be based on the Kengo Shogun, an adept wielder of the Katana, who bore the same name.
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Ashikaga Yoshiteru Boss Guide
In order to obtain this Weapon Proficiency, start farming at NG+ level 180 Missions.
His light attacks have minimal damage, whereas his strong ones can be evaded easily.
The best way to deal damage to Ashikaga Yoshiteru is to keep Blocking his attacks to deplete his Ki. When you have the opportunity, nab some light barrages and end with a ground finisher to hasten its decrease. At this rate, his attacks should not consume so much Ki, however if it does, simply evade them and get behind him to counter. Rinse and repeat for at most 4 times to finish him off.
When you defeat him, it unlocks Dual Sword Master Trophy.