Knotty: Difficult to understand or solve.

Intricate: Difficult to understand or analyze for having many interconnected elements.

Intimidating: Discouraging as a result of perceived difficulty.

Insoluble: Difficult or impossible to solve or explain.

Inflexible: Difficult to change or alter refusing to change one’s attitude.

Incomprehensible: Difficult or impossible to understand.

Impenetrable: Difficult to penetrate or enter impervious to reason, argument.

Herculean: Requiring great strength or effort.

Hard: Difficult to resolve, accomplish, or complete.

Hairy: Alarming and difficult hazardous (informal).

Grueling: Physically or mentally demanding to the point of exhaustion.

Grim: Difficult to consider depressing or worrying.

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

Fraught: Causing great stress or anxiety.

Fractious: Difficult to control unruly.

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

Exhausting: Difficult in terms or physical or mental exertion.

Exacting: Requiring much time and attention difficult and demanding.

Elusive: Hard to fine, capture, or understand.

Draining: Strenuous work that seeps away life.

Distressing: Causing emotional difficulty.

Demanding: Requiring much effort: physical or mental.

Daunting: Seeming difficult to deal with in anticipation intimidating.

Cruel: Uncompromisingly difficult without regard for others.

Convoluted: Intricate complicated Difficult to understand.

Confusing: Not easy to understand bewildering.

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

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

Burdensome: Causing difficulty or worry physical or emotional.

Bruising: Physically wearing and damaging.

Baffling: Confusingly or perplexingly difficult.

Back-breaking: Physically demanding labor requiring great exertion.

(A reference to the Augean stables cleaned by Hercules as one of his legendary 12 labors.)

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

Arduous: Demanding great effort and labor.

Ambiguous: Open to multiple interpretations difficult to interpret.

Abstract: Apart from concrete, practical existence.

Whts another word for previous