Neutral:
* Taskmaster: This is a relatively neutral term, simply meaning someone who demands a lot of work.
* Driver: This is a more direct term, but still fairly neutral.
Negative:
* Sweat shop boss: This term implies harsh working conditions and low wages.
* Workaholic: This term is more focused on the person's own excessive work habits, but can be used to describe someone who pushes others to work too hard.
* Pusher: This term suggests someone who is constantly pushing others to do more.
* Bulldozer: This metaphor implies someone who is forceful and overbearing in their demands.
More informal/humorous:
* Boss from hell: This is a common phrase used to describe a difficult or unpleasant boss.
* Captain Tightwad: This term implies that the person is stingy and only concerned with getting as much work as possible for the least amount of pay.
The best term to use will depend on the specific situation and the tone you want to convey.