I personally suspect that Savathun will have a role to play in defeating Xivu Arath. She may have even dropped us a hint on how to do it too. Before she became a Lightbearer, she allowed herself to be imprisoned in the crystal structure by Mara as they worked to remove her worm. We could, theoretically, do the same to Xivu Arath. Basically starve the Hive god of War from receiving her tribute by isolating her and let her worm destroy her.