The High-Spirit Demon remains the hardest mission overall I think (even if the boss is really easy), I remember it being a total pain in NG, and in NG+ I've been breezing but so far it's the only NG+ mission that gave me trouble.
You could likely even forge them, but not matter how they are generated, they are very, very rare. Highest level missions have the best chances of dropping them, but even there they are far far away from being a common occurrence.
It's weird, why is everything at level 160, except the rare 170? What happened to 161, 162 etc. Just weird that you can do 160+1, +2 etc. but then nothing between 160 and 170.
I also got my very first 170 drop in Calling on the Fire God, a level 200 mission. It was a weapon type I didn't main, but at least I can soul-match it. My Otegine+4 (water-imbued spear) s now level 170 with B- scaling in Constitution, by far my strongest weapon. O_o
Speaking of lousy RNG, the skill drops can kiss my ass. I got exactly 2 skill drops so far (one time in NG, and once in NG+ just now, that's it). At least the first one was for a weapon I did use, but the 2nd one was for a weapon I didn't main. Bleh. Why lock skills behind super rare RNG drops? Ridiculous.
As an aside, I apparently got the Plat yesterday. Huh. So ringing those bells did do something after all. I completely missed that you could ring them in NG (tried hitting and it did nothing, but I accidentally discovered that a strong attack did ring them when redoing the mission in NG+) and I was wondering what it did, it seemed kind of useless. I was hoping it'd weaken the boss or something.