punishing vs arduous

punishing and arduous both are adjectives.

Word	Noun	Adjective	Verb	Adverb
punishing	No	Yes	No	No
arduous	No	Yes	No	No

As adjectives, punishing and arduous are synonyms defined as:

punishing and arduous: characterized by effort to the point of exhaustion; especially physical effort

Other synonyms of punishing include backbreaking, grueling, gruelling, hard, heavy, laborious, operose, toilsome.

punishing (adjective)	arduous (adjective)
resulting in punishment	difficult to accomplish; demanding considerable mental effort and skill
characterized by effort to the point of exhaustion; especially physical effort	characterized by effort to the point of exhaustion; especially physical effort
	taxing to the utmost; testing powers of endurance