To receive this achievement, you must defeat the hidden boss Phantom.
To do this, you must first
find Phantom.
*** Spoiler - click to reveal ***Phantom's lair is hidden behind a breakable wall in Sinner's Road. Go to the top of that area and take a left into a room with maggot floors and swinging spikes. The left wall is breakable at the top.
This leads to...
*** Spoiler - click to reveal ***The Mist: This area is a randomized maze with a trick to get through it: If you find illuminated butterflies, you can play your song for them and they will fly towards the proper exit for that screen. There are around 6-8 consecutive screens that must be completed this way. If you get lost and want to leave, just fall off the bottom of any screen with a bottomless pit. NOTE: Exiting out the wrong exit will reset the puzzle to a different solution. As for the enemies in this area, the best thing to do is agro one at a time, wait for them to wail, then jump and downward slash. They do eventually die and give a bunch of silk to boot! After that, you will come upon the Exhaust Organ, the lair of Phantom.
There is a bench before the fight, so don't worry about having to do that last part every time you die. For the boss fight:
*** Spoiler - click to reveal ***I suggest tools that are thrown straight ahead or onto the ground. Heal in the air after Phantom throws their pin and down slash on your way down. The forward pin throw is also a good time to throw a tool when landing behind them. There are 3 phases to the fight. Remember that patience is a virtue in this game. Since Phantom tends to jump around, avoid down-slash pogo combos, going for the right type of slash based on what move Phantom decides to use.
I did this fight in act 2, but I'm sure it can be done sooner, and probably later.