If you enter a room where a stone grimace is (or any variation of a stone grimace), beat the room, exit the room, then re-enter, the room will shut and new monsters will re-spawn. I feel this is acceptable for the feel of the challenge.

Also, some monsters that don't fly, or jump, might get stuck behind rocks. First, if you don't beat a room within 2 minutes, all the doors open. Also, 3 dollar bill / fruit cake can create Ouija or ipecac tears. I am not worried that you won't be able to progress.