Hey Gamers! This achievement is for completing Veteran Ruins of Mazzatun and Cradle of Shadows, which are two 4-player dungeons in the Shadows of the Hist DLC.
Below you will find video guides that cover all boss mechanics for these two dungeons on Veteran.
Of note, the final boss of each dungeon is run on the Hard Mode setting, which you do not need to do in order to get this achievement. You simply need to beat the dungeon on veteran for the achievement; however these videos will cover all mechanics, including those of Hard mode.
Hope this helps!
-Damron OutCradle of ShadowsRuins of Mazzatun