First Soleil

You should wait until you get a soliel that isn't -atk. Mine is a +Spd
-Res and works great. -Atk isn't horrible but is one of her stats you dont want a bane in. She still reaches 50 atk but a neutral atk is alot better.

keep her just in case you get more, she's actually really good with an ATK boom. Mine has Firesweep Lance, Glimmer, Swift Sparrow and Heavy Blade.

Well her offensive statline becomes 35/38 instead of 38/35. I'd say she's still worth it with those IVs, not optimal of course but still serviceable. Soleil has plenty of builds, she can run her default Firesweep Sword with Life and Death(don't recommend Heavy Blade seal here cause she's -atk, although LaD still bumps her to 55 attack which is quite high), a default bruiser set with Slaying Edge/Wo Dao and Fury/Desperation, a more premium build with Distant Counter and Wrath or even a speedy tank setup with +Def Safeguard and Close Defense sacred seal(or even double Close Defense too). In case you're just looking for a budget set for Arena Assault purposes, she can still run Firesweep+Darting Blow quite effectively. If you think the -atk is too big of a problem consider Threaten Defense, or if you need to proc Heavy Blade more reliably, consider Threaten Attack too.