@MFauli:
Using powder against Fiend
I don't remember any episode where they said it wouldn't work. If it was something you could visualise you could burn it, but a fiend likely doesn't know about powdered weapons being thrown at him. (might blow up the canister though). Plus, the fiend was supposed to be downwind, so the powder travels faster.
Mirror
This was forced, I agree. The fiend did travel to Tokyo on a boat, after all. It's hard to imagine that it never once had the desire to look at itself. That red hair should give it away.
death feedback
We've established that the feedback is via hypnosis when they seal your cantus. Your brain "learns" to suffocate yourself when you attack a humanoid. Maybe since cantus users have the power to project their thoughts into the world, the idea that you suffocate yourself actually becomes real via your own mental powers.
What also doesn't quite make sense is why Kiroumaru still sided with Saki when they had the psycho-buster. His reason for abandoning the thought of killing humans was because he concluded that there is no way to upturn them. The fact that Saki obtained the psychobuster is proof that they now have the ability.. though I suppose it's not anywhere near enough to kill the entire human race unless the queerats learn to manufacture it. Their technology is still way off from doing that right now.
I don't know who I dislike more at this point: Saki, or Shun.