Omega f2p setup with Kidomaru in my opinion should include:

1) BT Sakon

2) Edo Kidomaru

3) & 4) Slot open for your imagination and ninja / summon completion in your collection

Why Sakon? Kidomaru scales pure taijutsu -> Sakon deals pure taijutsu so this duo should be able to drop very high damage together. Main Fire / Earth to get quickly chakra and last slot I picked Jugo since he is super f2p, and with Y mystery gives more standards which equals to scaling, some shields, crit increase / debuff removal.

EDIT: Kidomaru with 4 stars is playable, before that imo not worth even checking. (he gives every sound ninja 10% tai per their move at 4 stars)