Have you tried the lineups on Youtube? If you tried those but still couldn't, you can either wait until you increase your power, get BTs on certain characters (chojuro is one I know) just get lucky doing them. Ninja exam gets much harder if you don't have the right ninjas or even the characters. To avoid stressing over ninja exam, I wait until I gain a lot of power (usually about 50K) then try again and repeat the process until I beat the level.