Really cool game! Given that I've spent more time on (what I assume to be) the final boss than the maximum estimated completion time, I'm guessing I'm missing something. Even going in with full HP/MP, I still don't stand a chance. The best chance I can see is using coffee ASAP and then spamming Soul Mirror every turn to maximize our longevity, but that requires perfect RNG and still probably doesn't get very close, given how little damage output that gives compared to using the attacking spells.
Perhaps I was intended to save the Quiet Dream until this fight. Maybe another time I'll start a new run and try to speedrun to the tower while maintaining as many healing items as possible, but even surviving long enough to use them feels dubious.
But, I don't want my skill issue to take away from the fact that the game is really good. It's just not in the cards for me to beat it with my current understanding of it.