arduous

adjective

1. Needing or using up much energy; testing powers of endurance.

   “The movement towards a peaceful settlement has been a long and arduous political struggle.”

2. Difficult or exhausting to traverse.

3. Characterized by effort to the point of exhaustion; especially physical effort

   “worked their arduous way up the mining valley”

   “a grueling campaign”

   “hard labor”

   Similar: backbreaking, grueling, gruelling, hard, heavy, laborious

4. Difficult to accomplish; demanding considerable mental effort and skill

   “the arduous work of preparing a dictionary”