This takes place on Episode 10 - Legacy, and is attained by finding the hidden access only available by using the Grappling Hook behind the mansion grounds.
In order to find this area, follow the dirt paths around to the back of the mansion grounds, and you will find a small area with two enemies at the end of a walkway. Take them out, and equip a grappling hook. Now shoot the ledges that hang from the cliff side and climb up. This is unlocked once you reach the top.
*Posting for MAKA91, because he is only playing the Xbox One version.