I'm just speaking from experience. Fujiyama killed me a few times because I was being dumb, but Woo shat on me more times than I could count and a lot of that wasn't even because I was being bad. It was just shit RNG in the face of Mokou being a dick.
And when I say non spells, I don't just mean the last one. The sixth and seventh non spells are more difficult than all of her spells too. Extremely random and they can screw you over quite badly.
Going by your logic, Hainiryuun, the hardest spell is obviously Honest Man's Death though. Who the HELL can avoid that laser? If you don't put any effort into learning the spells paths and whatnot, HMD becomes impossible. And of course the same can be said for Fujiyama. I'm speaking from a perspective that assumes you have a way of handling every spell card Mokou has. If you don't put any thought into learning Fujiyama Volcano, then that's fine, but it just makes you inefficient, not the spell the hardest she has.