Stock up on as many points from all the shops as possible. It may take months, maybe even more, but one day we will get to the point where you will be able to buy some of the best ninjas in game via this points in star system.

As for what you should buy before stacking up on these :
Group shop - Only Medium refines. If you are just starting you may get anko 2 star (if she is 2 star ninja)
Arena - Nothing from here is really necessary. I'd recommend to keep these for yourself.
Ranking - You may want to get hinata, and low refine runes. Other than that keep.
Survival trial - You may want kabuto nothing else really.

Summon - You are free to do with these whatever you want, they are not used in star system later on :)
This post was last edited by TakafumiNaito at 2017-3-31 19:41