How to build my Soliel?

I'd personally just keep default Firesweep Sword with the Speed boon, that way she can make use of her high speed to actually double units without fear of retaliation.

I have 2 builds for mine but i find more use out of her firesweep build especially since mine is +spd -def.
*glimmer,Firesweep,LnD3,hit and run,D speed and heavy Blade
*glimmer,wo dao +spd,swift sparrow,desperation

Speedy firesweeps are a very good mix. Galeforce can also be useful, but only with Heavy Blade. A good set you could use is
+Spd/-Res
Firesweep
Draw Back
Galeforce
Swift Sparrow(Death Blow in its absence)
Drag Back or Swordbreaker(because there is always Ayra)
Threaten Speed
Heavy Blade Seal.

I have a +Spd/-HP Soleil with Slaying Edge+, Wo Dao+ (both are Speed refined), Brave Sword+ and her default Firesweep Sword+.

She's pretty flexible to build. Experiment with her and see what you like best.

Here's mine below.

A set that I've been using is a Firesweep Galeforce one, on a +Spd -Def Soleil. It's great, but it's fully player-phase oriented, so you really need to have your team structured around her. I recommend an Infantry Pulse user and a Dancer.

Firesweep S+, Draw Back, Galeforce, Life and Death 3, Hit and Run, Smoke Skill (Atk, Def, or Spd), Heavy Blade 3 seal.

She darts in, and instantly procures Galeforce against most enemies (because of two hits & Heavy Blade). Then, since the Smoke skills lower surrounding enemies, you can dive back in for another kill and use the Dancer to bring you to safety, or you can use it to fall back to safety and play a more opportunist playstyle.

First off +spd -res is 100% the most optimal IVs for pretty much any build with her. Second I simply suggest sticking with FS, easiest and arguably best build she has. Plus she can run Wrath with it and safely proc boosted Moonbows with ease