Difficult adjective – Requiring considerable physical or mental effort. Impenetrable and difficult are semantically related in obscure topic. In some cases you can use "Impenetrable" instead an adjective "Difficult". Nearby Word: difficulty
Problem Sleuth broke the impossible way back when they encountered a Solve the Soup Cans puzzle so utterly impenetrable and difficult that the protagonist was forced to reload from a save-state made after the puzzle was completed.