I wanna show off an old favorite as a little birthday treat for myself, hopefully if the story brings us back to Múspell Laegjarn can get a new fancy Chosen Hero form too. Even if she does I probably will still be using her original form as a main stay in every team I try to make

Weapon: Arcane Renegade - Her old PRF has been outdated for ages and ages, but I never wanted to use any of the other arcane swords because they looked so horribly tacky and edgy, a petty reason to be sure but eventually I got my hands on Renegade and it's been working wonders for her. Special jumping is a fun tool at her disposal and getting a boost in stats based on opponent ATK helps her to compete against lots of more modern units

Special/Emblem: Archrival/Edelgard - Part of me wanted Flamewall for flavor reasons here but haven't gotten any fodder to make that happen, Archrival gives DR just for existing though and let's her special jump by 3 if fighting a green foe so it's helping her get the job done. Edelgard being unconditional galeforce just helps her be as aggressive as possible or even let's her retreat using her full movement  and without needing to worry about canto control effects
 
Blue Skies 4 - Damage dealt goes up, damage taken goes down, a simple and practical A Slot to help deal as much damage as possible. I'm not sure if the flat 8 taken out of damage taken helps with Reflex, according to the wording it adds from other sources but E!Ash's description is the same effect but says it doesn't stack, hopefully smarter and better players can correct me on that

Patience - Does it still count as mixed phase of you force the enemy to initiate on you? It's an incredibly fun skill to use along side Reflex, and it's healing on hit stacks with Renegade, getting back 14 per hit can get back into fighting shape real quick especially since she has more than enough speed to trigger the potent follow-up

Deadly Miasma - Corrin's ring makes initiating a bit difficult and retreating to keep a unit safe just as hard too , nothing a little smoke bomb can't fix, giving her a a small stat advantage as well as chipping nearby foes helps too, and since she usually gets to initiate twice in a single player phase she can immediately make use of the Haze terrain to make her second attack all the more effective

Solid Ground 3 seal - Her Weapon and Patience are the bread and butter of the build, both rely on her having as much ATK and DEF as possible, her RES alhas always been her dump stat so this is honestly just the perfect seal for her

Tumultuous Echo - Extra movement and long distance Canto is great and let's her be super aggressive, flying in to hit the 2 most important threats before leaving everyone else in a bunch of Haze, using Canto to get out of reach to fly back to her team to reapply team buffs before the next turn

Years and builds may come and go but a favorite always remains