If the Witness him/herself is paracausel, how can we defeat them? And would we want to?
My take on it is that we won't fight the Witness head on but we will stave off their attacks in some way. I'm looking forward to seeing how this all unfolds in Lightfall and the Final Shape.
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I imagine that the Traveler or the Gardner would need to get involved in order for us to defeat the Witness. It could even be something fairly simple like the Traveler overcharging us in a way that allows us to do something in a new way or just in a bigger way than before.