|Name of the file: The Legend of Zelda : Ocarina of Time 3D [3DS] - Author: ANO - [3DS]|
After growing up but before talking to Sheik for the second time on the Time Temple to learn the Prelude Of Light, she will tell you that you can rest in your home to replay past battles. This allows you to fight any Boss at your house after they are first defeated during the storyline. To unlock the last challenge, which is all bosses in sequence, defeat them all in Boss Challenge mode.
You have to beat the game once in Normal mode.