Answer:

Abstract: Apart from concrete, practical existence. ...

Abstruse: Difficult to comprehend.

Ambiguous: Open to multiple interpretations; difficult to interpret.

Arduous: Demanding great effort and labor.

Augean: Requiring so much to complete as to seem impossible.

Back-breaking: Physically demanding labor; requiring great exertion.

Baffling: Confusingly or perplexingly difficult.

Bewildering: See baffling.

Bruising: Physically wearing and damaging.

Brutal: UNPLEASANTLY difficult.

Burdensome: CAUSING difficulty or worry; PHYSICAL or emotional.

Challenging: Difficult in a way that is usually interesting or enjoyable.

Complex: Not easy to understand or explain because of multiple interconnected elements.

Complicated: See complex.

Confusing: Not easy to understand; bewildering.

Convoluted: Intricate; complicated; Difficult to understand.

Cruel: Uncompromisingly difficult without regard for others.

Daunting: Seeming difficult to deal with in anticipation; intimidating.

Demanding: Requiring much effort: physical or mental.

Distressing: Causing emotional difficulty.

Draining: Strenuous work that seeps AWAY life.

Elusive: Hard to fine, capture, or understand.

Exacting: Requiring much time and attention; difficult and demanding.

Exhausting: Difficult in terms or physical or mental exertion.

Fatiguing: See exhausting.

Finicky: Difficult to please.

Formidable: Discouraging as a result of perceived difficulty but OFTENTIMES with a positive connotation.

Fractious: Difficult to control; unruly.

Fraught: Causing great stress or anxiety.

Frustrating: Cause something or someone to be upset or annoyed; prevent from achieving something.