I have long thought the flat 1-4 scale for villain difficulty really didn't seem accurate, so I though I'd try to create a better one. For now I'll keep it to rankings between 1 and 5, though I think there's a certain logic to saying that there's enough ambiguity in the categories to deserve a 10-point scale, so that Akash'bhuta would show as being more difficult than original Spite, even though the difference is too fractional to reflect in a smaller range. Since this is all experimental, I welcome feedback based on personal experience (NOT based on parroting the results of the statistics thread, as I don't think the methodology there is rigid enough to produce truly definite answers of what's actually objectively true, rather than what reflects the experiences of the limited pool of people who are contributing to it); if you think Ambuscade deserves to be a Difficulty 2, please tell us why, with as much detail as possible of how he's stomped you in the past.
Without further ado, here are my rankings for all current villains.
* Akash'bhuta: Normal 2, advanced 3. She's not very aggressive, but those 200 HP can take a long time to slog through, and a few Primeval Eruptions at once can leave you buried neck-deep in more Limbs than you can possibly kill. (If I get to the point of having these ratings adjust by H level, I'll need to reflect the fact that she's particularly awful for small parties; if you're down to one surviving hero, an Entangling Brambles is an instant game over for anyone other than Fanatic or the Argent Adept, as far as I can tell).
* Ambuscade: Normal 1, advanced 2. I just don't see how Ambuscade (in his current form, not the character as a whole - I'm among those who hopes to see him as a Vengeance-style mini-villain who does pretty much exactly what he's always done when solo) can ever pose more than a trivial threat. Even his Advanced mode only manages to make him a nuisance rather than an actual danger; I've never lost to him and have only had a couple heroes so much as incapped, even when I behaved flat-out suicidally during the fight. Those 50 HP just don't suffice to keep him alive for any longer than he can Vanish, not when he can't destroy hero cards and only sporadically deals damage.
* Apostate: Normal 2, advanced 2. Apostate is a "high 2" in my opinion; he's very hit-or-miss, going down easily in some games and being almost invincible in others. However, while he is sometimes more difficult than a 2 suggests, this has very little to do with whether he starts the game with a 3-HP "players skip their draw step" card in play; I don't think I've seen more than 2 heroes miss their draw before one managed to kill the Tome. His Advanced text on the flipside might matter, but in my experience he's hardly ever on the flipside for any length of time, as the heroes usually get all the Relics killed before taking him down (or use the Sucker Punch loophole). And I still think the text about the Tome raising Condemnation from the trash must be a misprint, because when is an 11-HP target with DR 1 going to be in the trash first, unless the Tome comes out later (which certainly isn't the case in an Advanced game)?
* Baron Blade: Normal 1, advanced 2, promo 2, promo advanced 3. Pretty much the standard baseline which defines the difficulty scale. Normal Blade is capable of winning a game, but only occasionally; I have only a little experience with his promo, but it doesn't seem to add enough to increase his difficulty by more than one tick, since he's still working with tiny numbers of HP and often playing cards which do very little.
* The Chairman: Normal 4, advanced 4. While I consider the Chairman the single hardest villain to beat, my own record against him fails to bear this out, though I suspect at least one of my victories against him was the result of a rules error. Thusly, I'm leaving him at 4 rather than making him the first villain with a base difficulty of 5, though I may change my mind on that. While a game with advanced text is certainly capable of being deadlier than a normal one, Chairman already has such sharp teeth that really, making him quicker to flip seems as likely to help as to hurt, simply because it makes it possible to kill him faster, leaving only a souped-up Operative to deal with. My theory is that if you lose the game because of Advanced text, you would probably have lost the normal game too, it would just have taken longer.
* Citizen Dawn: Normal 3, advanced 4. Pretty straightforward.
* The Dreamer: Normal 3, advanced 4. Probably should have a higher ranking than Dawn, especially with a high "H" number, but for now I'll stick with the written numbers.
* The Ennead: Normal 3, advanced 5. The advanced Ennead is just a ridiculous beating, so I'm currently opting to set them as one of two villains who jump more than one difficulty point when you Advance them. The difference between possibly having no more than three gods to kill and as long as you need to do it (assuming you have Visionary burying the Rise to Powers and so forth), versus having at least 4 of them by the end of turn 1 and a maximum of six turns before they're all out AND getting +1 to all their tiny damage-instances AND healing 2 HP each per turn...it's just a quantum leap in difficulty, and I'm confident that I'm right to call the advanced Ennead one of the two most devastating battles in the game.
* Gloomweaver: Normal 2, advanced 2, promo 3, promo advanced 4. I am perhaps being excessively generous by not dropping him all the way down to 1; the occasions on which he can actually win a game will be exceedingly rare (and usually involve Cursed Acolyte). But for the moment I'll give him the benefit of the doubt; somehow the writers of the rulebook thought he was a 3, and I'll pretend they were only slightly wrong. I'm mostly guessing about the promo version, and entirely guessing about the advanced promo, which I've never played; Skinwalker definitely poses a threat, but with just 50 HP he tends to be a manageable one, and once he's the Rotting God, he actively helps you kill him, so it's possible he's not as dangerous here either as I've credited him with being.
* Grand Warlord Voss: Normal 3, advanced 4. I consider Voss a more "solvable" 3 than Dawn, since none of his Minions have more than 3 HP or any damage reduction. He's still plenty dangerous, though, with Forced Deployment and the alternate Game Over perhaps making up for whatever he lacks compared to the Citizens, and so I'll leave them at the same rank for the moment.
* Iron Legacy: Normal 4, advanced 5. This is the only one of the existing 4s where I'm confident that Advanced status is a sufficiently hardcore upgrade to be worth reflecting in his difficulty level. It might not matter if you never reduce damage, but the odds that you were going to survive in that case aren't great; damage mitigation is just about necessary to survive against this villain, so being largely unable to do it is pretty damn crippling, even if your hero didn't have any cards that would have done the job (and thus no cards which are dead against advanced IL).
* Kismet: Normal 2, advanced 4. Kismet spends a lot of time doing not much (playing largely irrelevant Jinxes or the like), but also has bursts of extreme violence, especially on her advanced side where she can get as much as +3 to damage in a 5-hero game. This variability makes a 2-point split in her ranking seem appropriate to me, but I could be talked out of it more easily than for the Ennead.
* La Capitan: Normal 3, advanced 4. You are unlikely ever to convince me that LC's incredibly aggressive opening does not justify a 3 difficulty rating, even if it's possible to geld her quite thoroughly by getting the Ship sunk and the Crew locked down. I insist that her printed 2 is a typo, possibly inverted with Kismet since she's sometimes listed as a 3.
* The Matriarch: Normal 4, advanced 4. She's definitely extremely mean, but fractionally easier to handle than Iron Legacy or the Chairman (as long as you don't take +1 math damage), and the added difficulty posed by her Cohorts being indestructible doesn't apply if her Cohorts never actually show up, which is relatively unlikely given how quickly she tends to play her deck out, but it's still possible. Thusly, while Advanced does make her more dangerous most of the time, I'm inclined to think it doesn't make a full point of difference, though again I could be convinced to reconsider.
* Miss Information: Normal 3, advanced 4. While she's extremely frustrating, she's not superlatively difficult, at least not in regular mode where she has only 45 HP. Advanced mode gets extremely hard if you can't deal irreducible damage, but I'm inclined to think it doesn't quite deserve a 5 ranking.
* Omnitron: Normal 1, advanced 2, promo 2, promo advanced 3. I know some people have suggested that Omnitron is harder than the Baron by a substantial margin, I really don't see it from my own experiences; he occasionally manages to beat the heroes, but usually he's just a big target who occasionally distracts you with smaller targets, and only sporadically actually damages anything. I have only a single game against the promo version under my belt, so I'm purely speculating about the difficulty there.
* Plague Rat: Normal 2, advanced 2. I'm not increasing his difficulty in the Advanced game because, while he gets a bonus to damage, so do the Infected heroes, and thus it becomes actively easier to kill him. It's still not precisely an easy game, but it's quick and brutal and largely luck-dependant, which I figure well-fits the definition of a 2.
* Spite: Normal 2, advanced 2, promo 3, promo advanced 4. Spite is not a hard fight so much as a tedious and frustrating one, and Advanced text just says "don't even bother trying to beat me until I flip", after which it no longer has much effect. Either way, he's not especially difficult, what with being a single-target villain whose plays are often blanks, and who eventually turns into a predictable robot. The promo of course doesn't have that problem, and is definitely vicious; he should perhaps even be a 4 in his non-advanced mode, and/or a 5 in the advanced one, but that's just a guess since I haven't faced that version at all.
* Vengeance: Here the calculation becomes extremely variable; I'd say they're unquestionably never less than a 3 in total, and that bumps up to 4 if the Baron or Fright Train is in, and bumps up another one for advanced, but that might only be true if you have both Blade and Friction (the two who have cards affecting the entire villain party, as well as having a mean combo on Advanced mode). According to the Vengeance rulebook, Ermine + Friction + Proletariat should only be a 2, but I'm not sure I buy that, having played that combo once and not found it significantly less hard than Vengeance in general.
