You'd think water and ice could actually wreak havoc on them. Use magic to fill the cracks in thier stoney flesh with water, then flash freeze the water. When the water freezes in the cracks of the stone, it'll expand and bam. Your using the water like a wedge to break apart the stone. In real life rain causes damage to statues this exact same way. It rains and the water pools up in the cracks, then over night the water freezes... of course in real life it takes a while before any significant damage is done, but if you're using magic whose to say what could happen.

Also, on a side note, the deeper the cracks the better. So maybe you could use water/ice as a killing blow after you beat it up a little.