How to deal with a manager with unrealistic expectations?

My manager keeps rostering me on shifts with staff that are pretty bad at their job. She expects me to pick up the slack. When I cannot. She gets annoyed the next day - pointing out quite bluntly all the things I did not complete and she expects me to complete. But its literally impossible because I am not given good enough staff to help me on my shift. She is aware that there are a lot of weak staff so spread out the good staff to always be on shifts with the weaker ones. Somehow she expect the good staff to be able to overcompensate for these weaker staff. Its driving me crazy. How do I tell her to back off?

How to deal with a manager with unrealistic expectations?
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