arduous

characterized by toilsome effort to the point of exhaustion; especially physical effort

• “worked their arduous way up the mining valley”
• “a grueling campaign”
• “hard labor”
• “heavy work”
• “heavy going”
• “spent many laborious hours on the project”
• “set a punishing pace”

taxing to the utmost; testing powers of endurance

• “his final, straining burst of speed”
• “a strenuous task”
• “your willingness after these six arduous days to remain here” F.D.Roosevelt

difficult to accomplish; demanding considerable mental effort and skill

• “the arduous work of preparing a dictionary”