Sonya help

You can build her like Rhajat, an offensive horse slaying build. Or a BladeTome build. Imo, I'd rather stick with the Fury and desperation set as it has great synergy. Dark Excalibur has built in wrath in it so that's a bonus, pair that with Moonbow and QP seal she can proc Moonbow almost instantly.

I guess it depends on what you want her to do. Sonya isn't particularly amazing at anything aside from having a tome that deals +10 damage on Special triggers, so, if you wanted to, you could make her into a mage tank for annoying units in Arena Assault.

Sonya (+Atk/-HP)
Dark Excalibur
Reposition/Draw Back
Iceberg
Resistance +3
Quick Riposte 2/3
Res Ploy 3
Distant Def 3.

With this, she hits 41 resistance on enemy phase against a mage, 47 if receiving a +6 Resistance buff, and because her speed isn't high, she'll probably be doubled, allowing her to proc Iceberg on the first initiation if the foe doubles her. She has enough resistance on her own to reliably ploy quite a bit of units, and if you feel like the extra res isn't completely necessary (with the added +10 from Dark Excalibur), then Atk Ploy could take its place since it adds more utility to her for her team.

If you have enough (3) Infantry Pulse and/or Ostia's Pulse units in your barracks with a free Quickened Pulse, then consider running a first-turn AOE Special build with her. She is the only unit with natural access to Wo Dao effect on her tome, which is taken into account when you trigger the AOE Specials. I see this from time-to-time in my Arena runs, and Sonya with this particular set-up is really hard to deal with.

A sample set:

Dark Excalibur
Reposition / Draw Back (anything you want here)
Blazing Wind (or any other AOE Special you have, really)
Fury / Life and Death
Wings of Mercy
Res Ploy (or something else if running Life and Death)
Quickened Pulse

It is important to note that in-combat buffs from skills (like Blow / Strike skills, Spurs, Drives, etc.) don't get factored into the initial calculations, as the AOE Specials only care about Sonya's visible Atk - foe's visible Res. (This is why a skill like Fury or Life and Death is preferred.)

To reiterate: This set does require heavy Infantry Pulse support to pull off, but I think it can be used in Arena Assault or the in-game content to great effect. If you don't have enough Infantry Pulse to pull this off, then you could just settle for a lower cooldown Special like Moonbow, Glimmer, Iceberg, etc. Otherwise, a cookie-cutter Fury + Desperation or another build that the others suggested here will still work on her.

I have a +atk -res sonya in my ranks.

I gave her triangle adept and axebreaker along with glimmer.
Its a special kind of build which i use on some units if their spd is just too low.

Important is, that the unit needs as high atk as possible.
She can double the axes and kill them when her atk is buffed, afterwards her special is reday which happens to be pretty strong against blues thanks to triangle adept and her wo dao effect.

So she could one shot lancers with her special ready. In addition her res ploy helps her to one shot blues much easier. Blue tomes should be killed with her special again.

It always depends on the situation though, how much res the enemys have and how much hp.

Also your buffs are important. For that i would recommend atk tactic buff and res tactic buff. Both are easy to get from legault and l´arachel. Her res will get in a good amount to ploy most units if she gets the res buff.

atk 56
res 37

If you buff both. Its up to you to give her res +3 or atk+ 3 at this point. You basically have 61 atk if you consider the ploy effect on the units you ployed.