Post Update Solution:
This achievement is cumulative!
Earn a total of 2500 coins and spend them in the Monster Island store!
It was much easier before the fix but now it won't cost you any MS Points this time around, though you will have to grind out a fair amount of levels to earn 2500 in game coins, to spend in the In-game store.
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Original Version 1.0 Solution: Before the game was removed for the following..
NOTE: "240 total MS Points needed for this Achievement"
Buy "3" 80ms point Coin Packs from the store is the Cheapest way to do it..
Step #1 From the main menu click "Store"
Step #2 From the store click on "Coins"
Step #3 From the Coin menu Click the coin pack --> "1500coins for 80ms points"
Step #4 From the MS points page click "Buy" will cost you 80 ms points each time.
Step #5 repeat steps #3 & #4 two more times once you click buy for the 3 time from the ms points page the achievement will be unlocked when it takes you back to the game.