On the third island from the new scenario (the japanese flavored). You will be able to unlock explosive barrels right next to the catapult.
Buy some with three coins each and wait until the greed come. Usually after the 10th day the horde are big enough and one barrel will be enough to set on fire 20 of them. They are usually packed really near each other so it should not be a problem.
If you miss it, just try the next day, since the numbers of the enemies will grow, and your chances will be higher.