Other DUSK Guides:
This secret was actually very easy to find. Here is how you do it!
First off you will have to be on Episode 3 - The Nameless City, and choose the level "E3M8: As Above, So Below." The video shows you from the very beginning of the level where to go.
You will want to play up to where you get to the giant room where you are small compared to the map and fight a bunch of enemies. The next room will be this room.
You will find a Hallowed Health behind some bars. This is a secret area, but very easy to figure out.
You will notice the wall behind it is different. This is the passage to the other secret area (that doesn't count as a secret, so it doesn't go towards the secret counter." When you inspect the wall, you will get this message.
Shooting won't do anything, you need something much more, powerful. Go back into the main room, and jump down to see a little building.
Inside the building you will find the item we are looking for, the most powerful weapon in DUSK ;)
Pick this up and run all the way back to the wall, and throw it at it.
Pick up the gem (or walk into it to trigger the text/achievement) and BOOM! You have your new achievement! Congrats!