It took me nearly 24-48 hours total to beat ninja exams 130-180 with 65k power. Is becomes easy after more people clear it; only few people cleared it before so was painfully hard.

Use these three channels. You have to use different lineups for each exam;it is all about strategy

https://www.youtube.com/watch?v=NtvqxPy16Ow

https://www.youtube.com/watch?v=D6S8l72mDIU

https://www.youtube.com/watch?v=tp3SInqM3dY

Good luck :)

PS: you have a really good collection of ninjas that should last you for a while , focus on getting more power. Pick ninjas from recharge and spending rewards.