Here's a weird bug - Rain of Fire does not seem to work once a certain amount of a floor is cleared.
First noticed with only two balrogs, scrolls, and fountains left. Rain of Fire failed to "hit" the balrogs every time. Noticed again (on Floor 5) with about a third to a quarter of the floor remaining.
The spell seems to work if there's more of the floor covered, and I don't know the exact moment it stops working.