You can boil them, just do it at a very low temp. The heat from "boiling water" will break down the bone and make it brittle. Depending on how much meat is left on the skull, baking soda will get it off, no problem. Mix a little clear dawn dish soap in if you go that route to help degrease the skull. If you do not degrease, then over time the fats and grease will leach out and it will turn yellow.
On the beetles, I have several thousand in my old chest freezer, which is my beetle box. They clean up a skull in just a few hours to a day or so.
My favorite method is beetles, followed by maceration.