To get this achievement, you need to collect 15,000 coins from daily challenges in one month. You can see your progress on the Daily Challenges page.
Easy challenges will grant you 100 coins, medium challenges 150, hard challenges 200 and expert challenges 300 coins.
Also if you complete all three challenges on a given day, you will get 150 bonus coins.
If it's early in the month when you're trying to get this, just change your system date to the past month, and you will be able to complete all of that month's challenges.