To get this achievement you need to get to rank 25 in the game. Every time you complete a chapter in the game, you get $$$ based on how many kills you got, what kind, etc.
The only other guide here is only meant for people with the overkill edition of the game.(me, and thousands of other's I'm sure, are playing the free version available from April 16-April 30). It does not tell you how much money you need to earn to get there and I was surprised no information was here on this it.
But regardless now that I am rank 24 I see it takes $500,000 career earnings to gain rank 25. This is easily obtainable through two playthroughs of the game if you are going to going for all the achievements in the game. (EDIT: I just beat insane and earned roughly $600,000. I got the achievement at the end of chapter 36 of my insane difficulty playthrough)
You should not need to go out of your way to get this achievement, just be patient and it will come in time. My first playthrough on normal earned me about $275,000 just for anyone curious. A tip though would be to use overkill as much as possible because it significantly increases your money earned.
This guide isn't showing you an easy way to make money, just the requirements and rough timeframe it would take to earn rank 25 so sorry.
If there's anything I can do to improve this let me know