To further back up the point, the entire reason Arrancar are hard to cut, is not because they have naturally durable skin, it's because their reiatsu is so great that it creates a "barrier" or an extra layer of skin so to speak. the only way to get through that barrier is to have greater, more refined or condensed reiatsu.
Originally posted by Omega Vision
Wouldn't you have to prove that it's a spiritual attack first?
When Ichigo first meets Zaraki, he had a thought of something like "it feels like im being cut all over my body"... physically, there was nothing wrong with Ichigo, it was just the feeling he got from being next to Zaraki's reiatsu.
Same thing happened to Nanao when Yama released his, she wasn't being hurt physically, her spiritual being was being pushed down on to the point she couldn't even catch her breath.
Originally posted by Omega Vision
Or prove that it can actually harm someone of Diana's durability?
Zaraki, with a casual swing of his sword cut a building in half without actually touching the building... When he fought Nnoitra, a swing with more power behind it did nothing but create a spark. The fact that Zaraki couldn't initially cut him is proof that it's not just about physical strength... you need a greater amount of reiatsu.