depends on how and why, the general rules can be as followed

1. keep trying to see if you can find a pattern to help you win the fight

2. try experimenting with different teams, either on your own or with guides

3. Maybe your lacking a particular team or setup, maybe try again once you have it

4. could be a stats or difficulty thing, try again in after a bit of getting power elsewhere or waiting for the difficulty to be reduced if your a lower power on your server

This is the basic structure of dealing with Ninja exam following this pattern and repeating. This works on the mindset side, though particulars would be google a specific exam, such as 140 as an example, and see what power and teams they used to do it.