On the Kujo Kethba boss, my team kept dying to the invulnerability phase. Since we were going for a trifecta clear, we kept resetting and trying again. It wasn't until after several wipes and resets that we realized both DPS wore the Null Arca set. It's possible that light attacking the boxes would fire a shard at the main boss, who then reflects the damage back to you. A friend of mine told me that in DSA, one of the templar adds kept killing him until he realized Null Arca activated on it. I'm not sure if this is a bug but I'm posting this here for anyone who experiences difficulty in this dungeon.