Soleil +Atk/-Hp??

+Atk/-HP is still fairly good IV spread.

In regards how you should build her, it's mainly up to you. But the question should be: what does your team need and what do you want her to do?

Here are some build options:

Slaying Edge+/Wo Dao+ (Spd)
Reposition
Moonbow / Glimmer
Distant Counter / Fury / Swift Sparrow
Wrath / Desperation / Vantage / Renewal
Threaten Spd / Spd Smoke
Seal: Distant Def / Close Def / Heavy Blade / Quickened Pulse / Attack +3 / Speed +3

Firesweep Sword+
Reposition
Moonbow / Glimmer
Life & Death / Death Blow / Swift Sparrow / Brazen Atk/Spd
Chill Spd / Swordbreaker / Hit & Run / Drag Back
Threaten Spd / Threaten Def
Seal: Attack +3 / Speed +3 / Heavy Blade

Brave Sword+
Reposition
Moonbow / Glimmer
Death Blow / Swift Sparrow / Life & Death / Brazen Atk/Spd
Hit & Run / Drag Back / Desperation
Threaten Spd / Threaten Def
Seal: Attack +3 / Heavy Blade

More options the better so you're not limited to just one type of skill. Especially if you don't got your hands on some premium skills.

I'm confused. Your topic lists +Atk/-HP, but your text says -Spd. I'm not sure which way to suggest a build for her.

Soleil basically works with about any build you want to give her because of how versatile her stat line is. If you're building her for T20 play, I'd highly endorse the Armorsmasher (+Spd) over the builds the site recommends as she is one of the best Armorsmasher wielders in the game. She'll one shot even +10 H. Jakob. Slaying Edge also works well with her. Either way, I'd recommend giving her the Heavy Blade seal as very few units have more Atk than her.

For A skill, I use Swift Sparrow. Fury works as well. For a bait build, you can use any of a number of Bond options. Brazen Atk/Spd also can work as she rarely dies in a single round of combat. For B skill, I use Swordbreaker just because a lot of red units run counter-swordbreaker these days, and also even though she can keep pace with most every red to avoid getting doubled, she needs Swordbreaker to ensure doubling units like Ayra, Karla, and Mia.

You *can* build her for Distant Counter, but I personally think it's a waste on her (and most reds) in general as higher level play is starting to get pretty devoid of range units, and blue and green has a better assortment of range counters available.