Suppose You have to do a project of 50 hours work in only 20 hours. And You have to make delivery on Time. now what will be your testing approach

Opinion 1:
In this critical situation first analyze the business critical s and focus area of the product. And break the product in small chunks and prioritize them as per client prospective. Then start working on as per priority. It is also know as risk based. After that you will able to complete the delivery on time.
Basically this type of condition is not possible because everything would be predefined during the whole cycle. And in typical Agile terminology , first all cross functional team member , product owner and scrum master set the delivery time as a "sprint" for given product on given release time with the approval of clients.

Opinion 2:
You need to work on Project Strategy which involves priotity-based scoping and incremental delivery

Opinion 3:
As per stated by other members, you have to prioritize and determine risk. You should consider talking to the user and development teams when making this deliverable. You should consider what the user's priorities (expectations) are along with the stability of the requirements and code. If you do not get reasonable feedback, you will have to prioritize and provide a risk statement. It sounds like you are in a somewhat challenging position as what you are describing indicates that the customer is setting limited time lines and from my perspective this indicates that this was not well planned out and the customer did not work with you to understand your estimates. You should be able to follow the SDLC and test properly. That said, test approach is to indicate the tests that you will focus on requirements that have the greatest value to the customer (detailed tests with good coverage) and risk is that the other requirements (lower importance such as usability) will have limited coverage.