Here's mi recommendation for this achievement: SAVE CRYSTALS
If you own enough crystals you can buy a HUGE amount of coins which counts towards the achievement!!
In order to buy the coins you have to go to:
Store tab ---> Scroll down to the bottom and choose the amount of coins that you want to buy:
500 crystals gives you 100 000 coins
1500 crystals gives you 360 000 coins
4500 crystals gives you 1 250 000 coins
Personally I get my crystals from the Daily and the Crew bounties. The daily bounties gives you 30 crystals per day if you completed it and the crew ones gives you 120 crystals per week, so if you play every day and have an active crew you can gain up to 330 crystal per week!
I know it will take a while but this game is a lot of grinding
Spend your crystals wisely