Accidentally let the final boss get a copy of High Priest Thekal and play it. They were at ~250 armor and 1 HP.
First weirdness: Bringing the boss down to 200 total health (hp and armor) triggered the new hero power, but set his HP to 200 and kept the armor. So now he had 399 effective health. Oh well, guess it’ll be a grind.
Second weirdness: Managed to get through the armor and get the boss down to ~150 HP before he dropped C’thun. I expect the score screen to show ~150 HP left on the boss, but instead it shows 1.
Guess the game calculates how much HP is left on the boss by how much damage you did, not by how much HP is actually left. So I guess my next run will be easy?