1) Speak to client what is most important for him
2) Ask client what are acceptable within the short span of time
3) Identify the Functional Points(which can be done during Requirements Analysis)
4) Measure the Metrics w.r.t time and effort and come up with the statistics
5) Conclude the critical business rules, dependency which are to be met
6) Derive a decisive strategy to test them bye ensuring optimal coverage.
I don't find a need to author and waste time in Test Cases here...
Test Case Prioritize concept comes when application is in Testing Phase and Business Prioritize concepts comes only when in requirement phase or any change request comes.
If anyone have different thoughts on this please share