Question - Have you ever managed writing the test cases without having any documents? 
          
        
        Answer - 
        
Yes, there are cases when we have a situation where we have to write test cases without having any concrete documents.
In that case, the best way is to:
- Collaborate with the BA and development team.
 - Dig into mails which have some information.
 - Dig into older test cases/regression suite
 - If the feature is new, try to read the wiki pages or help of the application to have an idea
 - Sit with the developer and try to understand the changes being made.
 - Based on your understanding, identify the test condition and send it to BA or stakeholders to review them.