I have just realised that the 'partial transfigureation to create a steel monofilament' idea probably doesn't work. The problem is that in general, it doesn't actually require partial transfigureation - one could simply transform an object into a steel spike which pushes (rather than transfigures) its way through the soil, going under the shields and then up into your enemies' brain. If this worked, someone would have figured it out by now, because this requires no special knowledge. Therefore, there is some defensive shield which would prevent this attack, and presumably the Death Eaters have shields raised, unless they want to run the risk of a sudden attack wiping them all out at once.