slave driver 
noun informal
A person who makes others work very hard: she was far too easy-going to be a slave driver...
- He was a slave driver and really made people work.
- An athlete often needs more of a companion than a slave driver.
- Horror stories have been cited of unfortunate junior doctors being forced to work entire weekends on their own with no sleep or food by slave driver health board paymasters.
Synonyms
(hard) taskmaster, (hard) taskmistress, tyrant