Do not give the assigned cases! I really hate this feature as it is a terribly inefficient way to do things.
You should be casing others in the cases you struggled in.
Firstly, how can you give a case properly if you've never done it?
Secondly, how can you get anything out of casing someone if you've never done the case yourself?
Pick a case you know so that you can better deliver to standards AND so that you can further learn (i.e. look at what they do well and don't do well, how they approach the case differently, etc.)