Beast Makers - Metalhead
To find the key during the Metalhead level and the rest of the gems you'll want to glide down into the secret passage just before the boss. For more directions, look at my screen shot above - I am standing at the spot you will want to glide from down into the drainage area.
If you're still having trouble finding this location, my screen shot below shows you the whole picture of the area around me and where I am standing in the screen shot above. Use that for some guidance. Also note, if you're having trouble finding the chest that the Key is used for, in the second wide open area where you fight Metalhead (the same room he kicks the bucket in) there is a small platform/walkway to the left upon entering the room. Behind one of the sludge waterfalls is where you will find the chest.