1. Complete Mission: "Under Siege" (Private)
   
2. Complete Mission: "Bridge Failure" (Private)
   
3. Complete Mission: "Resistance Rescue!" (Lieutenant)
   
4. After completing "Resistance Rescue!", go right of the Base Camp to the Hunting Grounds, and cross the new bridge over the waterfall. Climb down the first ladder you see and traverse all the way to the right of the cavern. Roll right into the leafy wall here and you will fall into a secret area. Travel right and meet/talk with Digger, whose drill is broken. Now to help her fix it, complete step 5.
   
5. Complete Mission: "Mineshaft" (Lieutenant)
   
6. Go back to where we met Digger in step 4. You'll notice that the cave is now deeper. Go all the way in and talk with her again. Her drill is broken (again). To fix it, complete step 7.
   
7. Complete Mission: "Tribute Cave" (Captain)
   
8. Traverse back to where we met Digger in step 6. The tunnel is now complete, and it leads to the Secret Developer Room, containing all of the game developers of Mercenary Kings. Talk to the developer named Jonathan, standing at the base of the giant brain. Give him $75000 to obtain the achievement along with some special gun parts.

Full credit goes to: (+[__]::) Stovich whose 'guide' from the comments section of another guide on this topic provided pretty much all of the text for this guide (with minor editing).

Honorable mention goes to: =(eGO)= KelpTheGreat who had this to say:
"instead of using the comment space on a guide to lambaste the author, why not just create your own guide? Not that your detailed instructions aren't helpful, but more people will probably read them if they were in their own guide."

So basically I just went ahead and made the comments into a guide so that Stovich didn't have to. You're welcome everybody! (If you would like more/better credit for this guide or have anything else to add feel free to contact me and I will do my best to make it happen.)