To get this achievement, you need to collect 10,000 coins from daily challenges in eight months. You can see your progress for the current month on the Daily Challenges page.
Easy challenges will grant you 100 coins, medium challenges 150, hard challenges 200 and expert challenges 300 coins.
In my experience, the easy and medium challenges offer the best value, so go for them first before solving the hard and expert challenges, which can take up to 20 minutes to complete in some cases.
Also, you should consider getting the premium upgrade (1,49€ in Germany) which will double these rewards and will make the grind a lot shorter.
Also if you complete all three challenges on a given day, you will get 150 bonus coins.
You can change your system date to past months, to play that month's daily challenges.