In no real order here are some things that adjust your power.

Equipment - Clear Ninja Exam / Plot for the materials.

Group Skills - Attack / Ninjutsu should be the biggest boosters out of the stats, though go for balance

Summon Runes - Instead of upgrading all the runes on one summon to a high level fill all the slots between the three summons and upgrade them that way HUGE boost this way.

Ninja Tools - http://forum.naruto.oasgames.com/en/forum.php?mod=viewthread&tid=11691
This is a pretty well done and easy to look at to understand guide on where the ninja tools you may need are located pre-brothers farewell team instance. (32k BP) On the same note, if you get dups of existing tools take a look at each of the tools bonus attributes such as Critical, Combo etc to see which you prefer. And try to match nature types with the stones as best you can for a further power boost.

Inner Eight Gates - Very costly however it is worth doing i neglected it for the longest time but coming back into it earning Purple and Yellow/Gold runes do give me a pretty fair boost.

Mood - Something i also overlooked for a vast majority of my leveling it's extremely useful and could honestly be a deciding factor in battles, for example i had +11% HP on Naruto when i was attempting ninja exam 119 the other day. I bumped it up to +25% health and he tanks everything thrown at him giving me the win.
Personally it's up to you which mood you prefer per ninja however if you go by their base growth whichever is going to be the lowest by default won't matter if its what you take -5% out from.

Refinement of the various sorts. Equipment refinement, purification, rune stones. Save your refines for the events in which they give you extra credit so you don't spend as many.

Lastly. Battle armour, i strongly advise if you're going to spend coupons on anything make it the thread. Cloth is very easy to earn from completing strong approach but thread seems like you never get enough once you start needing medium quality. plenty of guides on team for all the mains i'm sure.

That should cover the majority of things with at least some detail.
Group summon is negligible for a boost but thats a group effort not something you can do.
i may of missed something but thats generally about all you'll really need.
Character star levels / Cultivation / Awakening is all pretty much standard just make sure you save up enough coins to strengthen your equipment on a per level basis.