"Premium Ninjas" won't be much of help in those exams untill you get the right strategy for each level- and that contain- building team with right set of skills that can block/steal barrier/take dmg/do dmg/shield/heal, being in right positions etc etc... all depends of each level's situation.

There's also that in most cases you need to attack in certain order, in right time, with specific attacks + you need some (sometimes huge) luck to get effect you need. Sometimes it's about chaosing/immob. certain opponent, sometimes getting enough crits to kill him round 1. There's many youtube videos showing how to beat each level, with all details, order of attack etc. Sometimes it's wiser to switch mains- small tip from me- earth main works pefect for lvl 140.

And just yesterday I was helping a friend (power 60+k BP, no "premium" ninjas) with those exams- got lvl 124- 137 in a hour and I can surely say that nothing changed in difficulity of those past lvl 130, as I was doing them on my own like 2-3 months ago. So, turn down that attitude of "I have such awesome OP ninjas and can't beat it- means they made it harder!", take some time watching yt videos and may good luck be with you.