I died soon after correctly answering the first available riddle. I loaded my save & went back to the riddler to find that it was already completed, though the game did not track it as so.
once you reach the 13th-14th level of the dungeon, all the baddies start to give you experience by the bucket. By the time I reached the 16th sub-floor, I was about level 30. A few sections later, level 50.