I started working towards the card backs, with the only ones left being the one for Heroic Tombs of Terror and the two Heroic Galakrond's Awakening backs, neither of which I've started.
Given that I am collecting them at a later date than most, the deck based ones have been a bit easier due to more cards being available; most I cleared with priest built around the Activate the Obelisk quest. Although some fights were still very challenging and required luck. Heroic Free Medivh stands out.
However the one that stands out as the greatest challenge (so far) for me would be Heroic Dalaran Heist. At least with the deck based challenges you only need to beat one boss at a time to update your progress, yet with this one in the last chapter you need to beat 12. And some of them are bonkers. For example Archmage Kalec's Dragonwrath hero power means that it's not long until Arcane Explosion becomes a 2 mana enemy board clear. And a single loss will set you back the 40 or so minutes it took to get there.
Fortunately I got equally luck with a combination of passives of Rocket Backpacks (Minions have rush) and Robes of Gaudiness (cards cost half), along with big minions, meaning I was able to react to their board every time they managed to clear mine with their powerful powers or spells - With the last 4 bosses you need to clear their board after every turn otherwise you'll almost certainly lose, and they often have the ability to clear you board and leave minions on theirs.
I also got lucky enough to have 3 copies of Ironhide Direhorn, as well as set one to always be in my opening hand though the friendly tavern. That means on turn 3, with rush and robes of gaudiness, it always gave me the ability to clear a minion with a 7/7, generate a 5/5, and clear another minion with that 5/5 if necessary.
However I feel that I got very fortunate with my passives, treasures and draws, and I doubt I could complete the chapter again without many, many tries.
So, I would put that down as the hardest challenge I've faced for a card back, alleviated somewhat by having a very fortunate run.
What are your guys opinion on the hardest solo card back to get?
Also, on a side note, how challenging are the Heroic Tombs of Terror and two Heroic Galakrond's Awakening card backs to earn?