This is a walkthrough for Pro Mode, New Game, Gamecube Version (PAL). This is neither speedrunning nor playing as good as possible, I just needed to have som.
Stay in the center and just fight, running into a house on the left for a grenade when you can. After enough Ganados are killed, a chicken lays the insignia key on the roof above the grenade house. Go near it to get a hook shot prompt, so use it and grab the key. Drop down and run to the right in order to unlock the insignia door.