Stratholme

Watch it when killing Balnazzar, I killed him too fast, not allowing him to switch to the demon form and I didn't get the achievement.

It's odd that they don't include Above and Beyond as a retroactive way to achieve this. You must kill Balnazzar, pick up his head, return it to Light's Hope Chapel, and then kill Baron Rivendare. It seems to fulfill all the objectives...

As of patch 6.0something you cannot get this acheivement. I repeat since they split the dungeon into 2 separate dungeons, each time you complete one (killing the end boss) and then exit and go enter the other side and kill that boss, the other boss kill is greyed out on the achievement page. It resets! I have tried this many times without realizing. I hope to save someone else's time.