Not so fast…
Outclassed? Yes, but obsolete? Not at all…
GEN4 is overrun with powercreep and damage mitigation skills for infantries (such as the green movement passives for sw/ax/ln users) but that doesn’t make her worse, and IS might have a new upgrade tool for our units.
Thanks for the tag @Pokyo and @NickofTime80, I’ll provide advice!
My recommendation? Go all out for her, this is mine:
While [Death Blow 4] isn’t as optimal because you can’t increase the damage of the AoE this way you can increase both the damage of the normal strike (rarely needed but useful), and mainly the attack check for [Heavy Blade 3], let me do a breakdown:
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[Dark Excalibur (+eff)]: it grants +10 dmg. for offensive specials, this includes AoEs. Also it grants her CD-2 at the start of the first turn, this is really useful for chained maps like in [Tempest Trials+] events
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[Draw Back]: personally what fits best magic units like her, this moves both her and the ally one square back to the direction you used it (ex.: if it was used from the left both units will move to the left a square)
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[Blazing Wind]: IMO the most efficient AoE because this produces an attack that deals [ 1.5 × (
user's visible attack - target's resistance) ] at the cost of smaller coverage than {growing} specials — [Blazing Wind] covers a 1-square radius in a “+” shape (5 blocks) area which is the best as it’s more likely to find enemies together like this, mainly in [Grand Counquests], [Rival Domains] and even some [AR-O].
The damage is NON-LETHAL, meaning the target’s HP will drop as lowest as 1.
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[Death Blow 4]: I don’t really recommend it, I have it because that’s what I have and I really favor her.
It raises attack by +8 on initiation however the buff is invisible, she won’t deal extra damage by special with this skill.
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[Special Spiral 3]: this is the main engine for this build and key to spam AoEs… with this skill if the special triggers before or during combat your unit gets -2 CD after combat (AoEs trigger before so each time it procs the skill will activate), [Heavy Blade 3] compliments her best because it’s her bigger stat and checks are easier to pass, her total attack needs to be at least one point higher than that of the enemy’s total, I’ll describe the flow of the build below
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[Even Atk Wave 3]: just a skill for some self buffing, blue buff of attack+6 on even turns (2, 4, 6, …)
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[Heavy Blade 3]: needed for her to spam AoEs as consistently as possible
Flow
This gives a general idea of how the build works, after this you should read the next sections since things may not go as expected without some planning!.
Let’s see…:
- You start the map and Sonya gets her [Blazing Wind] to CD=2 but either 2×[Infantry Pulse 3] from teammates or 1×[Infantry Pulse 3] + [Time’s Pulse 3] will bring the CD to 0 on the first turn, readying the special
It’s quite expensive to set-up but it makes a difference to have an AoE ready.
- Order Sonya to attack.
If her AoE is ready, the following will happen:
- Her [Blazing Wind] activates and deals an area attack before the combat begins
- The CD resets to 4 and combat begins
- She attacks normally the enemy, generally being able to finish it off
- If she passed the check for [Heavy Blade 3] during the finishing attack she’ll get CD=2, otherwise it’s going to be CD=3
- [Special Spiral 3] will activate and grant Sonya 2 special charges after combat
- In the best scenario which is generally easy to meet the special will be ready for another attack
Advantages
She can “double” enemies that prevents follow-ups, this is the simple way for me to explain it but unless she’s faster than the enemy it’s just the AoE activating and the finishing blow
AoEs are executed outside of combat, meaning that their activation won’t charge enemy specials in case her target has to be someone she can’t KO
Highish damage toward enemies together, remember the area:
(The bigger "X" is the enemy she's attacking and the center of [Blazing Wind]'s attack)
This is a great boon for dealing with the reinforcement spawns in [Grand Conquests] and [Rival Domains] since 4 enemies appear in an incomplete "+" shape, provided there were 4 or less enemies (at the end of EP)
Recommendations:
[Life and Death 4] is perfect for her, she has low DEF and her magic bulk will still be high, in return of the loss for these stats’ bases by -5 she would gain attack&speed stats’ bases +7, I wish I could have it but I don’t sadly 
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[Time’s Pulse 3] is great for an extra special charge, it’s very expensive however but it’s a good alternative, it’s a great help on {tactic} teams where you can have only one more infantry unit however.
If you take the previous recommendation you’ll need only one infantry unit for [Infantry Pulse 3], the best candidate is Ninian, we all get one for free and her HP is amazingly high, she’s blue too so it adds color variety.
Nowi is another great unit to bring along.
Either one can have [Sudden Panic 3] and/or a [Panic Ploy] seal to help Sonya with more damage and easier attack check passes
If you’re NOT willing to give her [Time’s Pulse 3] she should get two infantries with [Infantry Pulse 3], that’s far less costly but you shouldn’t use {tactic} teams because she can’t benefit from their buffs and the other infantries however (the other types may get blue buffs from {tactic} skills provided they don’t have 3+ mates with same movement type)
Disadvantages
Powercreep is making her hard to run her build specially with damage mitigation being more available and busted PRFs, be careful with the red swords you send her to!
Weak to [Vantage], the AoE runs before combat, meaning that before she can deal a normal attack, if the enemy has said skill and can counter her the enemy will attack first. Protect her! 
Investment is high, to make her build work at its best it takes quite some set-up
Experiences
Despite all of this I plan to take my own highest-investment recommendations to make her a purple-haired Goddess of sharp gales 
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I don’t regret any investment, she runs her build near efficiently, sure, some enemies don’t let her fully charge the AoE but they’re quite rare for me and generally she cripples down her enemies.
Let me share a few shots, I wanted to share her average performance:
Build+stats of that Eirika:
If you like her and are willing to go high on investment do it…
1 5 0 • B S T
…is no joke 
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No, you don’t need it, Sonya will generally deal pretty high damage with an AoE to greatly bring down enemies’ HP to 1 or near death points and her normal attack will still be good to finish one target. L!Celica will at least be an unneded overkill but it may help on abyssals due to stat inflation.