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 (Envy and Lynette), but if you focus on one and only one, you'll kill both of them. Lynettte attacks first with Envy off to one side: I prefer to take on Lynette so she doesn't get any lucky hits in, but you can go for Envy; your call, but be fast and furious.
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.
If you're having any trouble, try leveling up your character to at least 13, and buy items to boost your Strength nd Speed (like Bionic Arm and Speedy the Porcupine from the secret shop in the tunnel section of "Shopping District") to the max before attempting again.