Difficult and Gruelling

Difficult adjective - Requiring considerable physical or mental effort.

Gruelling and difficult are semantically related in hard topic. In some cases you can use "Gruelling" instead an adjective "Difficult", when it comes to topics like arduous.

Nearby Word: difficulty

Gruelling adjective - Characterized by effort to the point of exhaustion; especially physical effort.

Difficult and gruelling are semantically related in demanding topic. You can use "Difficult" instead an adjective "Gruelling", if it concerns topics such as arduous, exhausting.