Okay, so last week was a good one for me: 2 out of the 3 I wanted to pure-clear done (and I got robbed on the third, I swear—lost my harpies, they would not respawn for what felt like 50 turns, and so despite having 1k life in Xochi and Bladewing saved up I eventually died to magic-skulls-from-sky bull-honky).
I scrolled through over 750+ posts on the early days to see if it was even worth talking about beating Hall of Guardians or Crypt Keepers, but quickly realized back then everyone was just scrambling to figure out the mode in general — times have changed 
And now, with epic tasks and medals, it’s really a lessened challenge. So I think these insights might be helpful for those late to the party to see what the current minimum thresholds (assuming I don’t suck 
) are for some of the formerly “impossible” or “very hard” factions.
Crypt Keepers first: no insight besides get lucky except to say that I entered the final boss room with only two Lady Moranas and still pulled out the win. 3 Medal of Seasons — that’s all it takes, with luck.
Hall of Guardians feels really good to beat, on the other hand—it’s not all just luck, and is one of the few factions where using the entire team is actually worth doing: Silent Sentinel up top to be a silence threat—but more for the ability to heal the troop you’re buffing attack to; Ethereal Sentry, so as not to be mana-blocked; Arcane Golem for Stun and for board control—be sure to use it to drop extra turns from 3x3 block removals; Gargoyle because damage and because the bottom slot is the safest, in my experience. I used the lantern banner.
With 3 Medal of Seasons equipped globally and Epic Task stat boosts (and probably Kingdom Power / Renown Stats we didn’t have in 2018), getting to the boss room is basically 90% possible. Even Lamia rooms can be beaten, but they’re the hardest in my experience. Bullettes are annoying, potentially, but it’s possible to be reshuffled into your proper order if you avoid allowing them to devour you (kill the troll quickly).
I didn’t get around to elite-medalling any of the troops, but I was so close to beating the boss so many times I really was planning to do so after Guild Wars troops this week (which of course is/was my priority). I think it would be beneficial for most of the troops on the team, as any stat is helpful thanks to percentage-based damage reduction and spell damage output based on attack.
Anyway, the basic strategy, stats aside, is to avoid skulls at all costs to be able to buff Silent Sentinel once or twice. Makes a huge difference. Take the skulls and be silenced if you have to—but that ensuing enchant might kill you, so really try to avoid it.
Likewise, avoid letting their Gargoyle cast. Take the mana before they get it.
Then just try to tank with Silent Sentinel, and use other troops opportunistically and/or to maintain board control. Gargoyle and skull stunned troops, ideally, and Ethereal-buff yourself anytime you don’t want to take a move on the board for fear of bad drops.
Sometimes it is better to use your Gargoyle to reduce armor so their Ethereal Sentry doesn’t work as well to boost Gargoyle, but this is a sucker’s gambit unless you have an amazing board. The explosions are dangerous to employ, anyway—only use Gargoyle to kill, or as a Hail-Mary, as a result.
Good luck, everyone. I go to kill more harpies soon, For the Horde
EDIT: Forgot to mention both Sword’s Edge and Whitehelm are Level 15 for me 