An easy level to get this on is the level 5 squares straight up from the start (2nd level up in the first column into the 2nd block of levels). You’ll have to clear all the levels in the first block to unlock the next block above. Once you get to this level, throw a rock into the helicopter on the right and it should set off a chain reaction that kills 4 enemies. If it didn’t work, throw the rest of your rocks away so you can restart because once you clear the level you cannot replay it that I’m aware of.