Hello,
These ninja exams are meant to be challenging across the broad spectrum of players, not easily beaten for all players. They are supposed to be extremely difficult even if you have discovered the exact recipe of ninjas, talents, and strategy. Since all of the exams have been cleared already, this demonstrates that they are doable. Have you considered looking for strategy guides or assistance in the strategy section, or maybe via online videos? That would tell you in advance if there are multiple waves, as well as what ninjas to use.

As for not having the right ninjas, that is unfortunate, and I do sympathize, but it may mean that those exams simply aren't doable at present. Not all exams are meant to be done at all power levels and all recruit-levels. You may need to come back once you can tackle them with the appropriate ninja team, or find a good substitute! Again, the strategy section is a great place to find advice for substitutes that can work.