On the title screen, input the code for the Power of Love Sword:
. WHen you hear the chime following the sequence, you'll know it works.
Play the game until you beat Stage 3. When you go back to the map, go back to the beginning of Stage 3, and wait until you see the big "GO" and Return to the Map. Then, you go to "The Main Stage" and use your Sword to beat the mid-boss WITHOUT taking any damage - there will be 2 mid-bosses, but if you focus on one and only one, you'll kill both of them. After this, you'll have to break your way through the Backstage Maze (I did a jump kick followed by two rapid hits of
to take down each door. Once you get through, provided you never took a hit at all, the achievement should pop just before you start your fight with Todd, the stage boss.