lineup =/= power, with exception of ninja stars.
Group skills are easy to miss, and you also want to spend your coupons more on power, than on ninjas. If you see event, and see a ninja that you "can get, but maybe use", and "sth for power", get the power item.
Class has nothing to do with power really, all classes have similiar overall stats as far as I am concerned. In summons you want to make sure all your slots are filled with runes, and spend summon coins on runes, not summons.
At this level you should already have all magatamas at lvl 3 or higher, if not, work on that. Are you at your limit in Ninja Exam? If not, get as far as possible, using teams found on forums, if needed.
Is your Battle Armor rank 2-3? work on that, it helps a lot. Also, try to get it to medium cloth as soon as possible, and then start working up slowly.
Eight Inner Gates are maxed? I mean, all slots with purple/gold/red chakras?
I believe your entire equipment should be purple/purple +1 by now, get on this if it's not
Just in case, awakenings. Cultivation runes. these two give lots of power as well, so make sure all are maxed.
I am not sure, but if I overestimated lvl 57 a little bit, forgive me. It's kinda difficult to remember what did I have at that time XD. You could help a lot by listing all the sources of power you know of, and how much they are progressed. It's easier to see the problem with it when you see it.
If I missed anything, also forgive me, it's middle of a night for me, and I might have forgotten a few things out of being tired :x