From Longman Dictionary of Contemporary Englisha hard taskmaster/mastera hard taskmaster/mastersomeone who makes people work too hard → hard
Examples from the Corpus
a hard taskmaster/master• The summer is a hard master.• True to his word, he schooled her in horsemanship and was a hard taskmaster.• He was a hard master and a dangerous enemy.• She was a hard taskmaster but a considerably fairer one than la Belle Ethel. 