Stygian Kastellan attack group

During the Stygian Wars the Dark Mechanicus often took the field alongside their Stygian allies.  When they did so attack groups of Kastellan robots, typically three led by a cybernetca datasmith would form the vanguard of any attack.  Imperial commanders often watched their infantry burned out of fortifications with ease before the robots went seemingly berserk and smashed apart any obstacle, be it wall, vehicle, or man with ease.  Often times it was only with heavy firepower these attack groups could be stopped.  Even then numerous heavy weapon emplacements were lost when shots bounced off the robots refractor fields and rebounded back upon the Imperial positions.