Also, I love how good Spire 1 is at testing how good your deck does things. It does it with elites, obviously, but bosses are probably my favorite examples here. The first and third acts especially. Act I rewards you for having your deck be able to do a particular thing, while Act III punishes you if you're over-reliant on a particular thing or set of things:
ACT I:
Hexaghost tests your ability to scale. You either need enough attack to kill it before it nukes you with a 6x6 attack or some way to survive past that without too much effort.
The Guardian tests your ability to attack or defend consistently, depending on which mode it's currently in.
Slime boss tests your ability to burst attack damage, but also your AOE capabilities.
ACT III:
The Awakened One punishes you for being too reliant on powers.
The Time Keeper punishes you for spamming low-impact/low-cost cards.
Donu and Deca punish you for having a deck that takes too long to scale effectively.
I think someone could probably write a whole book on the oodles of amazingly good design in that game. And I would read it!