The same like all the other challenge-based achievement.
For a diamond you need to collect 25.000 points. For this amount you need to literally complete all the challenges in a month. Since there are achievements tied to this anyway, this should come "naturally" as you collect your medals, earn XP and complete games.
If you want to get this quicker, you can buy a premium subscription as this will give you more coins per completed puzzle.
This achievement isn't hard, it is only grindy.