Outside of Naruto meta the only very strong and f2p friendly team is probably:

1) Edo Tensei Release Madara (4star)

2) Edo Tensei Sasori (4star pretty easy to farm in treasure - don't recruit him before you pull him 2 times tho!)

3) Edo Nagato (4star) - pretty cheap and farmable in recruit ninja

This trio with max skill trials is very powerful setup and it's very f2p friendly.

Other option is to save ~140k coupons and buy 6Path Pain when he is added to World Tree in the future