For the record, the current (that is, in-development) challenge ratings look like this:
EASY: Shotgunnery, Max Carnage, Pacifism
MEDIUM: Berserk, Marskmanship, Red Alert
HARD: Light Travel, Impatience, 100
VERY HARD: Confidence, Darkness, Overconfidence
BLADE: Purity, Masochism, Humanity
These are based on "casual" play. Stuff changes a lot depending on difficulty setting and badge collection, but we had to place the bar somewhere. Also please note that it assumes the average difficulty of expected runs in the challenge: this is why Confidence and Overconfidence are pretty high, even though start-scumming will eventually give you a very lucky edge. Pacifism is similar to that extent, but I've found that a lot of HNTR Pacifism games are winnable in spite of errors, and the simplicity of tactics makes it that much easier for a player to learn and achieve victory.
I wouldn't be surprised if these change a little over the course of 0997 (due to a number of gameplay changes). I wrote these in before we started beta testing, however, so you can assume these to be a more accurate listing of 0996.