Only the Lonely
To unlock this achievement you must acquire the best kind of diamond from spots where it has a Co-op activity sign. The easiest one that I have found (credit goes to Wreggitt) is when you get to Molten River chapter 4-2. The diamonds in Acts 1,2,3 do not have this diamond.
Just get to the point in my video and do what I do and you will have this achievement in minutes. Its extremely easy. I even show the map if you want to know how far you have to go and where it is located.
Video was done by Mike Kioski