This page of our guide to Ghost of Tsushima has a description of the battle with Yasuhira Koga - this boss appears in The Heavenly Strike side mission.
Yasuhira Koga will attack you when you defeat all the opponents. The walkthrough for that quest can be found on a separate page.
Yasuhira Koga isn't a difficult opponent. All you have to do is watch his quick attacks.
Try to knock him off balance. That way you give yourself a chance to hit him with some heavy strikes.
It is also a good idea not to give him any time to rest. Just keep hitting him to deplete his stamina and kill as quickly as possible.