Example : Worked their arduous way up the mining valley.
A grueling campaign.
Hard labor.
Heavy work.
Heavy going.
Spent many laborious hours on the project.
Set a punishing pace.

Synonyms : backbreaking, grueling, gruelling, hard, heavy, laborious, operose, punishing, toilsome

Example : His final, straining burst of speed.
A strenuous task.
Your willingness after these six arduous days to remain here.

Synonyms : straining, strenuous

Example : The arduous work of preparing a dictionary.