Bones Forest - Finish the level after killing 10 Headless Corpses, without being grabbed by them a single time
Use holy water, kills all the skulls and makes all the corpses pop up out of the ground dazed for you to grab them. You can get 1 or 2 (possibly 3?) each time. There are 6 corpses the first time, then you seriph jump the gap to get the gem and there are enough to give you the 10. just roll around and use holy water, got it first try.