This achievement looks confusing but actually an easy one. You may have noticed the words "Barrel, barrel, Zombie, barrel. Barrel, barrel, Zombie, crap!" and that's just the way how you can get the achievement. You should click on items/monsters in the order of

Barrel
Barrel
Zombie(Any monsters with green hearts)
Barrel
Barrel
Barrel
Zombie
Finally Crap(The items which give you $1 each)

Note that auto-click by system itself in this game won't be counted in, so you are indeed free to find your targets. I suggest you to hunt this achievement in a Maze Type map in freeplay games after you beat the final boss. Personally I think there're more barrels and craps you can find in these maps.