This achievement is awarded for getting 10,000,000 GMP. It is unmissable and due to the immense mass of content and hours you will put into this game it will most likely unlock while you are in the middle of the game with lots of hours and missions still ahead of you.
Just to be clear:
You DO NOT need to have the sum all at once on your bank account. This is a cumulative and will unlock as soon as you hit the mark in the game.
A good way to farm some quick money without doing anything is sending your troops on some of the combat missions. Usually the ones on the top will give lots of money to you (money you certainly need for some of the upgrading ;-).
Other than that I played many side ops to farm some money and work on the achievement for the side ops at the same time.
Keep ineterrogating your enemies to find stacks of diamonds. These will help you to speed this achievement up.
Personally I got it after 14 main missions and 40 side missions. Just to give you some kind of orientation.
If you need one particular mission that can be used as a quick farming spot, refer to the video below. Mission 4 can be done in 3 minutes if you have the landing spot for the helicopter unlocked.
(Video credit: MIM94)
Hope this helps!
Any improvements? Let me know in the comments below!
