On the Secret of the Sands level, where your in the pit where you have to close the 8 trapdoors, the foxes keep coming out when you kill them, until you close the trapdoors. You should use the Iron Belly technique to fight them (I recommend upgrading to maximum, if you haven't already). Killing them with Iron Belly gives you more coins than killing them with regular attack moves, and you can keep getting more and more until the desired amount is reached.