That is a lot of info I think need to made more digestible in the game somehow. Especially unblockable attacks and the like, I don't think I saw any special indicators for those and thus had no idea what defense to use. A tooltip/guide the player can revisit would be good too.
It's either the 40k hp boss or drillgamesh that dealt the 600k dmg. And regarding the debuff, I had just beaten the 40k hp boss (I had like 40k+ dmg at around 3.5k rpm in this fight iirc), then decided to chose accept punishment after the round, and I remember the text saying something about halving the drill power, then the next boss I went against was drillgamesh and my damage was in ditch (definitely muuch weaker than half power, weaker than how I was at start of game).
Revisited game briefly (it's nice that it saves), and damage feels like normal again (not buffed, but not in the ditch where it was when I left off). Also I chose atone with blood and ended up with negative hp and it let me continue.