Their shields drained as fire, lightning, and ice rained down along with the boulders, the Armatures had no defences beyond their metal hulls, which was being chipped away by arrows enchanted in a similar manner.  Regardless, the Geth forces were still advancing as were the ones that had taken damage.  The ones destroyed however melted and dispersed into nothing, a defense measure against being torn apart and studied by their enemies.  Their memory cores, one of the few things that couldn't be melted down completely, was wiped clean in the process.  The shields they generated gave them complete immunity to normal arrows unless they were hammered considerably, but magic seemed to be helping as more and more fell.  Even so, the Geth cruisers returned and totally blew away the iron gates in one pass, as well as giving their allies a way in as two more Armature shots made the opening bigger, but thankfully their accuracy was still impeded or they would have went for the guard towers instead.