Doomsday is pretty much made of entire mass, with no vital organs to hit. He does have bone protrusions and muscles, but I've read H/P and I believe it said something about him being made entirely of solid mass. Bertron created him to be nothing more than a living weapon comprised of solid mass and power, with no vital organs for weaknesses and the capacity to store solar-radiation for many thousands of years, thus creating the ultimate biological weapon.