This page of our guide to Ghost of Tsushima has a description of the battle with Kaede - this boss appears in The Legend of Tadayori side mission.
Kaede will attack you when you get to the place pointed on the map. The exact location of this place can be found on the page dedicated to The Legend of Tadayori side quest.
Kaede fights with a sword - it will be hard for you to move away from her. Don't expect to be able to attack her from a distance. You have to face her in a melee fight.
Focus on avoiding her attacks. She's fast and aggressive. She will take advantage of any mistake you make. Remember to keep your guard and attack only when you are sure your opponent isn't planning to attack.
Take advantage of the moments when she loses her stamina and lets her guard down. Attack her immediately with a series of slashes to quickly deplete her health bar.
Remember, this fight won't end with your opponent's death. Once you deplete all of her health points, Kaede will stop the duel and start talking.