Really enjoyed the boss! It is hard enough and nails better the "unfair" vibe compared to the older version, while completely beatable once you get the hang of it.
The damage system is very nice once you get it, but it can feel a bit confusing at first.
For the balance, I felt like the character upgrades for damage/defense were better than their counterparts from "minor upgrade of card without curse" on the current build. I suppose it would even it out with a longer gameplay, where you could create a build to privilege multiple low-value cards - or something like that.
The cursed cards balancing was nice!