In adventure,mode use any character to complete the mushroom kingdom stage but with the number 2 in the seconds,after that you will see a short clip of luigi knocking off the balance of one of mario's upper cut jump(b+up),after that you will battle luigi,go through the rest of adventure mode without getting a "continue?" and at the end you will battle luigi again,if you beat him he is a playable character.play adventure mode again and when you get to the battlefield stage you will battle both metal mario and metal luigi.