The Legendary Player is the toughest in-game Shadow Link that you can fight anytime. In order to unlock him, you must first complete all 50 streetpass challenges by Gramps in Kakariko Village. Afterwards, talk to Gramps and open up the challenge menu. Exit the challenge menu, and Gramps will tell you that you are the second one to finish all of the challenges. He will then offer to let you fight the first one who completed them all. After defeating the Legendary Player, you can challenge him again by talking to Gramps and opening up the challenge menu then exiting it.