Join an active clan save up the crystals from doing the weekly challenges and also the personal daily challenges buy the coin packs for crystals when you have enough this will count towards the coin achievements.
You get 30 crystals a day from personal challenges so 210 a week .
And 120 a week if you complete all the clan challenges
So each week you can get 330 crystals.
500 crystals will get you 100000 coins.