How to build a Rebecca?

You could Brave Bow her but tbh her attack even with +Atk is kinda underwhelming. She comes with Silver Bow+ so tbh as "meh" as silver weapons are usually seen she could totally be fine with it as it has lots of might. You can go something like Silver Bow+ Speed Refine, Life and Death, Desperation, and a C slot of your choice. Run Luna as special, and then Ardent Sacrifice you can use to easily drop her to desperation range! Hope this works well for you, she's not gonna be as strong as typical archers but this could be pretty decent.

Giving her LnD + Desperation + Moonbow/ Luna works fine. If +res she can run fury + iceberg or glacies. But at the end of the day she really is a below average unit.

I'd go for Firesweep+Swift Sparrow. You can run LnD but she risks getting 1-shotted especially since you have -res superbane.

The other skills, probably Guard and Savage blow/Breath of life. The idea is, she safely deals damage, heals your allies, without triggering specials from the opponent. You can choose to run a breaker skill, but with Swift Sparrow+buffs+speed seal you shouldn't have much trouble doubling.