Just a couple of notes before we start. This can only be attained on Hard Mode, it can be done by any of your living characters and you can get the achievement without actually digging up the treasure as all you need to do is find it. Everything you need to complete it can be obtained in Part 4, but I suggest waiting until Part 6 to go through with it. I used Delores because her story arc takes you to the Occult Book Store where you'll find a shrunken head near the counter. Pick it up and look at it in your inventory to discover it's Sepp the Navigator (pick up the eye drops on the counter while you're here as this will get you on your way to the 'Mean Person' achievement) Be sure to travel to the QuickiePal and pick up the bear repellant on the counter as you won't be able to get this without it. At the very beginning of Part 6 use your map to travel back to the mansion, head outside and you'll find a rusty shovel, pick it up also. From here travel to the Trailhead, use the bear repellant and make your way into the Spooky Forest. Make sure you can see Sepp's head in your inventory as it'll guide you where you need to go. Take it nice and slow because some paths are close together and the head will angle towards the correct one. After a few screens you should come across a unique area with a plaque on the right and a notice board to the left, on the ground in between you'll see an 'X' that marks the spot. Simply look at the plaque and you'll pick up a tidy 50G. Now to grab that rusty shovel and get diggin'. You can now take your treasure on over to Park Arcade on B Street and have some fun with it. Enjoy!