The witness is seemingly an evil speaker who wields the dark, and uses it. He didn’t create it and he didn’t come from it as far as I can guess from in game lore such as lore books and the throne world memory missions. This means we could end up draining him of his power, or taking out everything around him so he has little power left to use. Even if this is all wrong and he is the sole darkness whatever , I’m sure if we have enough light we could just golden gun him through his huge mega mind forehead