arduous

adj

1.
characterized by effort to the point of exhaustion; especially physical effort
backbreaking grueling gruelling hard heavy laborious operose punishing toilsome
Derived from:
arduousness
2.
taxing to the utmost; testing powers of endurance
straining strenuous
Derived from:
arduousness
3.
difficult to accomplish; demanding considerable mental effort and skill
Derived from:
arduousness