Exactly what it says. Play through the whole game as Scott, defeat the final boss and the achievement should pop. Difficulty won't matter and you don't need to worry about game overs, just get through the whole game as Scott. If you want to make it a bit easier, enter cn_Xcn_Xcn_Xcn_Acn_Bcn_Y on the title screen and listen for the chime to confirm you entered it correctly. You'll start every level with the Power Of Love Sword and make quick work of most enemies in your path.