This achievement is for eliminating 10 beetles. Beetles first appear on level woodpile XVII. This level has 2 beetle on so just rinse and repeat this level 5 times.
Points to note: you must finish the level completely as killing the beetles then restarting the level does not count towards your total.
Also the beetle must be killed with the same colour flame as the beetle is, fortunately this level has a red ant(to start a red flame) and 2 red beetles.