I think Heroic Scarvash is the worst of the worst, although Heroic Chromaggus and Heroic Thaddius are both totally abusive as well. I know you don't have Wild cards, but Molten Giants are 11, not 20, on the non-minion turns, making them very playable early.
Plague of Murlocs is a great Turn 3 play, as is Devolve(I know, Wild) against Scarvash.
I don't think, even with a full collection, a 50% Win Rate Deck against Heroic Scarvash is possible. I'd love to be proved wrong. And, current standard only? I'd be shocked if a 25% win rate deck was possible. Again, would love to be proved wrong.
I can consistently get 75-80% winrate with this deck. It does have some wild cards, but it can be easily replaced with other standard alternatives (such as Evasive Chimaera for early game, Steel Rager/Senjin Shiledmasta for a defensive 4-drop, and any solid even-cost late games such as Bolderfist Ogre)
The key is to have solid even-cost minions and solid early game spells such as Subdue. Paladin happens to have lots of suitable cards for this boss. Want to stablise the board ASAP, and put late game finishers once things are stable.
Another reliable alternative is to use dragon priest, though that might rely on some wild dragons.
That's a great set of lists. I don't have Hemet, Beardo or Shirvallah. I think your Mecha'Thun list may be able to be improved up given the current Wild Mecha'Thun decks using Dark Skies. Regarding Scarvash, Devolve and Plague of Murlocs are incredible, so I'm wondering if Powermace can replace Rallying Blade, and be built in a similar fashion. And Walking Fountains could be the "Shirvallah".
Thanks. Those lists are made before Descent of Dragons, I would've put Evasive Chimera for sure in Scarvash if it was done now. Hemet or Shirvallah can be replaced by other late games.
I've tried Paladin, Priest & Warrior for Scarvash, which I think are the most reliable classes against him. Thought a bit about Shaman but haven't tried. Won't surprise me if it turns out to be reliable, although I doubt it because Overloads make it harder to curve.
For Steel Sentinel, interesting idea about Dark Skies as well, worth a try but I guess poisonous minions are still more reliable.