Another clarification: Not all RNG is bad. It can be put to good use. Pure RNG is plain awful though. If the only thing that is determining whether you succeed or not is a dice roll, then it's just not fun.

To elaborate on what I mentioned, you could have two different rooms. One is the normal room, the other is booby trapped. It will apply a penalty to your group. Time reduction, Poison (can't clear), Paralysis, Reduced Healing. Etc. This makes it progressively more difficult to continue clearing. Think FF7 Battle Square.