Found a bug:
I just unlocked the puzzles and am now working on the last one of that first batch. That's the one where you get to play with the petrify spell. I was a bit stuck on the puzzle, so I tried the petrify spell on the thing I presume is the boss (the level 3 "The Firstborn" that kills you in one hit)
It got petrified. Then I went on to kill the rest of the monsters. After the last one was dead, I got the congratulatory message that told me to kill the boss and that it couldn't be petrified. There were no monsters left, and I'd petrified the boss, so the level got stuck.