-- Build a long, skinny campfire all around the rock, as close as possible to the rock, so the hot coals are right around the underside of the rock, and the flames lick up and around it.
-- Let it burn there for a few hours, so that a lot of the outside of the rock gets really hot.
-- Throw a big bunch of cold water on top of the rock ... enough so that it runs down the whole outside of the rock.
The thermal stresses that suddenly propagate through the rock, on the surface and for some distance into it, crack a layer of some thickness into several pieces, and the pieces fall off onto the ground.
If the cracking and crumbling doesn't extend far enough in, then just pick up the pieces of the layer that DID crumble off, and then go through the whole same process again, with the inside smaller rock that remains.
