As a fellow f2p, allow me to give you one of the best avice I got.

Stack position 1.

You power up all members at the same time only if you are p2w.

BP is mainly from refinement and cloth and rune stones.

Do Purification ONLY after your equipments have been refined to a high enough lvl, say lvl 8.

Charms also play a major part.

ps. I know you didnt ask this but in terms of ninja, Han really helps since alot p2ws are running flower guard with wind main and root with lm, and obviously em has shields.
:)