As the description says, this can be optained in the Horde Modus of the game.
The goal is to open a golden door.
Golden doors are doors, which are locked and get opened for money.
You will recognize them, because there are "animated golden hands", which want money from you. Inside the room you will find loot etc.
I found one of the door in the Horde-Level "Ausgrabung". This is the german title.
I dont know, what the level is named in english, maybe "Excavation"?
From the start, turn right and just go around the level on the outside and you will find the door.
It will cost 2500.
Once you got the money, open the door and the Achievement will pop.
Easy 15GS because 2500 is not much money.
I personally optained it in the second wave.
Have fun and pls excuse my bad english.