Soleil +ATK -SPD

Maybe wait for a better IV Soleil since she is quite accessible. 32 speed for a sword unit is not that great tbh.
But if you desperately need some good sword units then that can do as well. Just cover her speed with some seals or summoner support, you can even build her based on QR since she has decent defense. Another viable option is brave sword Soleil, but imo it is not a practical idea because most strong greens has very high hp and def so she might fail to secure kills sometimes without buffs.

I would go ahead and promote her now. Give her a Brave Sword and Swordbreaker and laugh as she kills almost everything.

Pick up a better IV later when she appears on a banner.