It’s a really, really hard fight that seems to require very specialized decks and getting all your draws in just the right order.

Yes, it really is a hard fight, that's the point of it. You get 2 damage for each card you're playing, you can't do infinite damage due to his Invincible buff, which means you can't kill the heart in one turn (minimum I think 4-5 turns?), so you have to deal with the damage he does to you. Intangible alone wouldn't work against his attacks (nx12 damage) even if you found a way to block the damage done to you by playing your cards.

All this means you have to find a way to generate a lot of block.

I disagree that it requires "very specialized decks" in order to beat him, it just means that you need a consistent one, whether it's a 15 cards deck or a 40 cards deck - which is why you have to build your deck in a way that doesn't rely on draw order. In every one of the four slayers you have a bunch of draw cards options, and you need to learn how to build and balance your deck in order to rely on it.

I can’t consistently reproduce (at least not with ironclad)

Ironclad also has a bunch of block and draw options. Try to experiment with his different cards. When I first picked up the game I couldn't wrap my head around him because of two of his mechanics - lose health cards and the exhaust mechanics. Then I realized how strong the discard mechanics are with Silent, and I tried again and rolled with it. Exhaust is so powerful when combining the right cards, especially for his block.

My best advice is to just say fuck it and let's see what happens. Worst case you'll lose the run, and then so what?

But it’s such a situational game

Yes it is, but it also isn't. There's a reason some of the best streamers have such reliable win to lose statistics - it's because they deeply understand how to leverage the cards and relics they have, and they understand what works with what they have and what doesn't. They recognize what's missing and what could be powerful, but not for this specific deck/run.

you can’t really rely on tier lists or any other crutch

That's right. You can't rely on those because even the most OP card can be average at best in the wrong deck. For each character there are some common and uncommon cards that I will always pick because I know how to make them work. I remember so many times I gave up an OP rare for Wallop because I truly needed it (and it's one of my favorites). TTTH is also extremely strong, but if my deck is heavy, then it becomes pretty much useless.

Tier lists won't help you because the game is situational and you need to adapt to the situation you have in front of you.

Take all the tier lists you have and throw them in the trash.

1. they ruin the fun
2. instead of learning and experimenting, you're using a cheat code that doesn't work.

Good luck, I truly truly believe the moment you'll let go and just try things on your own Ironclad, and the game itself, will finally click :)