Magatama - I've gotten one of my accounts to pretty high power early on, without paying anything, merely because I was lucky in recent events and got lv3-4 magatama.

Look at growth rates - Stack certain magatama and equipment on ninjas that have the highest growth with that stat.

To Strengthen - Show up at 9tails and always have your strengthens maxed, as well as your group skills.

Be in an active group - Group skills will give you a lot of power, and an active enough group will be able to upgrade skills within the first few days.