The Legendary Player is the strongest Shadow Link that you can ever face. To unlock him you must first complete every one of the 50 streetpass challenges by Gramps in Kakariko Village. Once done, speak with Gramps and open up the challenge menu. Exit the challenge menu and Gramps will mention that you are the second one to complete all of the challenges. He will then offer to let you take on the first one who completed them all-- the Legendary Player. You can challenge him again by talking to Gramps and opening up the challenge menu then exiting it.