Easiest to do in the Challenge Level Braaaains, took only 5 minutes, just unbuilt all the Towers and collect the coins (you'll get the Space Cash Achievement by doing this as well), exit and repeat, you should get the achievement when you start up the level the third time.
Not sure if you have to end the level, but I ran into the zombies with all the kids after collecting the coins.
So if you want to get this cheevo fast, just repeat this level
P.S. To unlock the level you can skip through the main campaign until you unlock it, if you want to.