The Mirror Dungeon puzzle that this achievement refers to is accessed via two sets of trapdoors at (X=110, Y=295) and (X=150, Y=290) in western Cysael, near the cliffs above the beach. One suggested way to solve this puzzle is to send two teams of two characters to each trapdoor. After going through the trapdoors, each area will contain a set of elemental pillars and an inscription describing the order to use the pillars in. Using the pillars in one dungeon in the correct order will cause an enemy to spawn in the other dungeon. Killing it will will drop either a round or square amulet. Using these amulets to bring down the barriers at the end of two passages reached from each underground area will unlock this achievement. (thanks to Niv Holson for confirming)
Note: If you already solved that Puzzle and did’t get the Achievement, just go back there and place the Amulets again and the Chivo should unlock!