Crown Trick

This achievement is currently unlocking for defeating a Black Monolith rather than its intended way. Take advantage of this before it gets patched because the way it's supposed to unlock is during Dungeons of the Deep (very hard endless mode) which is unlocked by completing the game.

I was able to unlock this in Day 2's Gears of Sorrow chapter while exploring randomly. I walked into a room and had to defeat the Elite enemy Black Monolith. This was the first time I've ever encountered the Black Monolith so I don't know for sure if his appearance is tied to the achievement. After defeating him the achievement unlocked.

This is what the room looked like on the dungeon map.


This is what the room looked like where the battle took place.


And this is the encyclopedia entry for the Black Monolith.