Just cleared floor 90. I have to say that I found 51-60 and 61-70 easy. 71-80 turned vicious. 81-90 is, for the most part, easier than 71-80, BUT:

Flame Dragons.

Good God flame dragons. They see you through the wall from the next room over, and kill you THROUGH THE WALL from the next room with an almost always one-shot fireball. (And if you survive the 1st one, they'll often immediately shoot another.) I guess there's a warning marker on the ground, but there must be a glitch for some people because I never once saw it.