Easiest way to grind this achievement is go into the "Fight" section at the bottom of the screen and just set bounties on all players. It doesn't have to a big bounty either. 100 coins is the minimum bounty you can set.
100 bounties * 100 coins = 10,000 coins
So it doesn't need to be an expensive or difficult objective to meet. Should only take 30 minutes or less as long as you have the excess coins to finish it.
The only problem appears to be that a certain point you might run out of opponents/players to place a bounty on. If that's the case, you can wait a little while for more players to appear to finish up placing bounties on.