Fallen Celica

Celica’s attack Rivals that of LA!Hector so she is a unit that is able to run glimmer effectively since it will do more damage than moonbow. Also having a low skill charge allows her to fight multiple foes by killing them in a single hit. Celica is a great unit for killing high priority threats even being able to duel winter tharja and have enough fight left in her to beat other green units that remain. Using draw back and drive attack make it so she can play safe and still be useful after killing her first target as she needs to pick her fights carefully. Alternatively you can use reciprocal aid to swap hp and keep her at full when a teammate isn’t useful in battle to reset her passive and activate escape route and desperation skills.

This build is unique and focuses on tanking/dealing damage and debuffing enemies with Panic Ploy. This build is surprisingly effective during enemy phase the most because of Panic Ploy. F. Celica Becomes an absolute monster and destroys almost anything once she's able to proc Aether. You can also run Ignis with this build to go even further beyond with her damage output. The only thing I've ran into that stops this build completely is if Reinhardt gets to her first and Wary Fighter Effie. There's more that can but they have a tougher time dealing with this build for F. Celica.

Beloved Zofia has the unusual property of self inflicting 4 damage to Celica after combat. When combined with fury, this becomes 10. This enables her to drop into desperation range after any combat regardless of if she is hit. Heavy blade is an excellent choice of seal as a a neutral unbuffed Celica at full hp achieved a total attack of 58 with fury, this making heavy blade extremely effective at activating high cooldown specials such as Aether.

This build's main focus is to create situations in which Celica is able to kill entirely in the player phase. Speed in this case is the best boon as when combined with fury 3 she is able to reach a strong 41 speed stat allowing her to avoid virtually all double hits as well as being able to double many units. Desperation and Ardent Sacrifice are both personal touches and can easily be replaced with either swap or reposition and chill speed based on your team composition. Heavy Blade 3 is not a requirement however if you do remove it you will want to trade off Ignis for either Bonfire or a Moonbow. If you do not want to use Heavy Blade it can be traded off for a +atk, +spd, or +def sacred seal as those are her best stats and would appreciate the boost. The C slot is completely your choice and is only left at Hone Atk as it is the cheapest option. Overall all of these are interchangeable however I do believe that Fury is her best A slot, if you do not have one to spare the only other skill you can really use to budget for it are Life&Death (since it is on a 4* now) or Triangle Adept to help with armored units. These do reduce her bulk a bit but should still work for her.

Even with an Attack bane, Fallen Celica is still formidable and hits like a truck. She requires little investment when it comes to SI.

I gave her Death Blow because she's mostly a player phase unit anyway. And with the Attack bane, she'll need the extra +6 Atk. Chill Speed is absolutely monstrous. It allows Celica to double the fastest unit on the field or enable another unit that is faster than her to double them instead. Savage Blow is there to get some extra damage off just in case she can't kill, although she most likely will. Her Atk stat is 48, but with the Attack +3 seal, it goes to its neutral state, which is 51. Add that with her Beloved Zofia's boost of +4, Death Blow 3, and a boost from Hone Atk or Dark Aura, and watch the heads roll.

(I should also mention that you'll need a healer to keep using her Beloved Zofia's effect. I personally prefer Physic+, but do what fits your playstyle)

Here's the math for both buffs:
(Hone Atk) 4 + 6+ 4 = 14. 51 + 14 = 65
(Dark Aura) 4 + 6 + 6 = 16. 51 + 16 = 67
Again, keep in mind that this is with an Attack bane.

So if you actually bothered to read all of this, you're a legend. Long story short, don't be disappointed with a -Atk Celica. She is still viable and useful.