Play through a World and reach the boss without getting hit.
Like "New Challenger", in a multiplayer game you can hold to block while your co-op partner does the work. If doing this method, try on Stage One or Two, where enemy attacks are less complicated and easily blockable. If going solo, use your Power of Love cheat and wait until you are properly leveled up to attempt to do this.
A common exploit is to load up the Third Stage (after you have beaten it a first time) where you watch Clash at Demonhead arrive in their limo. Watch the cutscene, and a few seconds after the "GO!" prompt disappears, exit to the map. Now move ahead to mid-level where you battle Lynette and Envy. Here, you will have to defeat them without getting hit. With your Power of Love sword and max strength this should be fairly simple, although each character has some moves that are hard to evade or block. You can play in co-op and have your friend do the work, but beware of Lynette's robot arms or Envy's kicks. Block, or move away if possible so your friend can attack.
Once you defeat these two, you then will have to crash through the walls to make it to the Bass Battle Mini-game. If you're a low level character, you may not be able to break through the walls before Todd catches up, and he will attack you then. Defeat Todd in the Bass Battle to take the fight outside. Once outside, the achievement unlocks.