i just came across this while trying to solo flawless but if you complete a dmg phase right before he attacks into a battery it will drop part of his shield. if you go through the encounter like usual and get his shield down he stays immune no matter what you do. i tried ragebaiting him into a battery, didnt work. i tried to toss him into both of the shock chambers and using the shadow grenade on him, didnt work. so you are softlocked if you let harrow destroy an active battery like milliseconds after dps phase