You use exacting to describe something or someone that demands hard work and a great deal of care. The Duke was not well enough to carry out such an exacting task. demanding a lot of effort, careful work, or skill = demanding
having complicated nutritional requirements; especially growing only in special artificial cultures; "fastidious microorganisms"; "certain highly specialized xerophytes are extremely exacting in their requirements"
severe and unremitting in making demands; "an exacting instructor"; "a stern disciplinarian"; "strict standards"