I got this achievement in-game, rather than paying real cash.
First of all, it should take a 1-2 day grind. Cemetery Catacombs and the Sniping missions (you need to by a sniper rifle first: about 5,000 coins) are the best to harvest from. The missions start off slow, like 75 coins and 50 exp, but when you reach the end it will be like 800 coins and 800 exp on round 32max (Cemetery Catacombs); 500 coins and 500 exp (Sniping missions) on round 20max.
Mission "Cemetery Catacombs" can be found (same mission, two locations) at the graveyard East of Socorro Town in Oregon and at another graveyard in Prosperidad in Arizona. Shotguns (maxed) are the best for this, but I have been using the Telsa Gun, which might be a wise investment (about 28,000-32,000 coins) as you go for the 10,000 enemies. A "win-win."
The sniping missions can both be found in Arizona and Oregon. The one in Arizona is located on a high cliff on W of the map. Oregon is at Assassin's Creek on a little step deck. I prefer the one in Oregon because it's simple: 21 targets with 7 per round with over 20sec to complete. Don't shoot the damsels. If you kill them: game over; graze them: skips to the next round.
And yes, the missions are endless or rather the last wave of the mission will be available to grind.
If you're crazy like me, I would make the Witch Impaler (150,000 coins) the main goal. There's a mission that requires it, and I completed all available quests for the other achievement (there's a big difference between missions and quests). The only quest I haven't completed is "Winter is Coming" and the only thing I think that is preventing it from opening is either future updates, that witch mission, or rank/level.
Please leave a comment if you know anything about best ways to grind and/or "Winter is Coming."