Purify is just a multiplayer. While important, stand alone it does not a lot.

Best value (before end game 150-200k+) Cave Key rebate (get at least 1050 spins for Gold Essence) is the way to go. With the new grocery shop you can get those 1050 for free.

Summon Cultivation and Magatama you do whenever possible, but you don't spend coupons on them.

Refine is another great boost. Make sure to follow this guide:

https://forum-narutoen.oasgames.com/page/show-post-30164-1.html

Myoboku training ground and battle armor (especially the cloths) are another great way to get power, but those are not easly accessable to low-casher / f2p.

Charms are also very good.