You will get this as you go through the levels. At the end of each Level (4th floor of each part) is a boss. This achievement along with "Heart of Stone" and "Immortal Beloved" (provided you did not die during the game)will unlock once you leave the Mansion.
Mansion = Bats - Beat them by lighting 4 fireplaces.
Attic = Stone Monster - Beat him by destroying his minions using the Eye of Sour-On (obtained during the attic level)
Basement = Giant Spider - You have to navigate around the maze in order to find the 4 sections the spider is in. Make sure you have the Flaming Branch (obtainable right before the battle) to beat him.
Catacombs = Zachery Graves - To defeat him, you will need to use the Skull of Enlightenment, the Eye of Sour-On or the Flaming Branch to beat his minions, then pick up the Hand Grenade and go near him and press A to set it off. Do this three (3) times then run straight up to exit the mansion when it starts shaking.
**Note: Don't bother stopping on the Zachery Graves level to find anything while you are running out, you cannot search.