If your Slave Driver is completely unrealistic about what you can handle, or is generally unaware of what you're experiencing, become an educator. Explain what an optimal workload would be in your department and what the effects of overload are. Then work with the Slave Driver to schedule work in a way that you both agree is more realistic and practical. Before that happens, gather data about how the quality of work deteriorates when people are overloaded. Remain positive and helpful.
Begin by comparing the Slave Driver's expectations and your job description. Write down what you and your staff must do on a daily basis. How heavily does the extra workload burden you and your people? What are the financial and operational consequences of constant overwork? Note what the best working processes would be for you and your staff and plot out a reasonable range of workload fluctuations from high to low. Figure out how to tell when overworked people are reaching the point of diminishing returns and beginning to make costly mistakes. Discuss all this information with your boss and jointly write up a plan to re-schedule the workload in the best way possible.
Distribute work evenly among staff members. Monitor and document how much is being accomplished, who is doing the work (even if it's just you), and how much time it takes, so that the Slave Driver understands exactly what is required to accomplish what is being demanded.