Legendary Lyndis

Send her home.

If you love her to death, build her. If you don't really care then give her her vanilla set and let her rot in the bench. I also have one, and never use her...

Do whatever you want.

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Although people rate her as a really bad unit, especially in tier lists, I just don't listen to them. I would personally try to make her the best I can, since she does have an interesting skillset, considering she's a fast bow user that can work on both phases. Use her base skillset, and just swap Desperation with Vantage. Give her something like Distant Def, and you've got yourself a nice little anti-tome unit. Pair her up with units that can teleport next to her, and support her stats. I would treat L!Lyn like a green tome unit, that just uses physical dmg. For specials, I'd use Iceberg, so her res stat can be of use, and will make Vantage much more scarier if she just so happens to have Iceberg ready, and it's the enemy phase.

Congratulations, you get another archer to use and give stat boost to anyone with wind blessing. I personally would change her A-skill to something else like Deathblow and give her Iceberg or Glacies if you're going to give her the Flashing Blade seal.