Rate my Cordelia

I guess it's a fair build.
However, the problem with Defiant skills is that they require 50 % HP threshold, which is very risky to attain, especially with Cordelias's frail defenses. An attack that might bring her under 50 % might as well kill her in the process. Galeforce is something you'll want to capitalize on with a +Spd Cordelia, even though it can work well on a neutral. And she will never KO a decent Def green unit, whatever her IVs, with a Brave Lance, aside from flier buffs. The seal seems a bit odd but I guess it could do the job when paired with Desperation (although a +Spd nature or flier buffs can free you the spot). Then again, a Cordelia under 50 % HP is probably dying on the next few turns if not particularly taken care of. Here's my Cordelia with a similar build for comparison.

Well defiant Attack might be good in paper but you have many issues like:

She needs to be under 50%,works as a buff so panic ploy hurt her and cannot reach higher Attack, despite would work with desperation her Speed is not that High to quad actually She would get doubled instead.

I suggest you db3 as her a skill since she will always have +6 when She attacks First, doesn't required hp to be lowand She still be buffed, true you Need to Attack First but you would always do since she won't Tank anyway especially with her defences and her low Speed. She Need to kill on her First 2 hits anyway so lower hp is hard to get and is not worth It anyway plus ad her +Attack Seal and you know you would kill almost everyone asside some Green tanks or wary fighter.

As for skill hit and run or draw back are better since she can escape especially with galeforce. As her c skill Ward flier is good It you use her on a flier team if not other buff are fine too. Still her vanilla a skill:ta is still good It you know you have to get a rid of pesky Red and you Can potentially Tank some hit from them (still use that on tt mostly).

Its cooky and I like it :D

Galeforce is cool!
Brash Assault + Desperation is cool!
I hope you have Reposition fodder so you can give that to her too!

I'll be a bit less picky than the other responses, but It really is worth addressing that there are better A-slots than Defiant ATK.

Cordi's speed gets her doubled on a lot, and her defensive stats don't compensate for that.
So, she gets one-shot a lot and you'll barely ever be alive to make use of Defiant ATK.

Defiant ATK is letting her down.
Death Blow is the A slot the meta tells you you should use, but you could use Life and Death, +3 ATK, maaaybe Fury? at least that'll help you safely get down to Desp + Brash Assault range.

An expensive option you could consider is Heavy Blade! Cordi's strong and with her high ATK stat Heavy Blade will get those Galeforce procs.

I appreciate the drive to make unique unit builds so I'll support this thing 100% :)
I also see what you're going for with the build, she's a beast in low HP situations, however I think you'll find that this build isn't practical at staying around 50%HP all the time.

Anyways, keep at it! I can't wait to see her in the Arena :D