This is my first solution (don't hate me too much)
This achievement is unlocked after killing Conjunctivius. (One of the possible bosses)
Location - Insufferable Crypt
You can find the Insufferable Crypt after the locations Ancient Sewers and/or Ramparts.
Boss Fight - The fight itself is not too difficult. 6/10 in my opinion. The boss has to phases, which it switches back and forth from. The first is its main form, there will be a few platforms to stand on and his attacks will be a body slam. (there will be an arrow in the direction he is going to attack in), this is pretty easy to dodge. His other attack sends bolts across the screen, it doesn't do a lot of damage but it's hard to avoid.
In the next phase, he will send tentacles. The best way to kill these is to dodge often and use ranged damage. Firebrands are pretty useful here. These tentacles get progressively harder throughout the fight.
After switching through those phases 2 or 3 times you should be awarded with loot, blueprints, and a shiny new 20g!
I recommend checking out Conjunctivius's lore at (
https://deadcells.gamepedia.com/Conjunctivius )