Help for -Spd +Res Elincia!

That's basically the easiest way to "fix" her, the other things is to build a team that focuses on her and give the ally unit Ally buffing skills and the best team for her is Flier emblem (obviuosly)

Though I also Recommend to give her fury instead of LnD cause she has really good res for an offensive unit, so I recommend to take advantage of it so she does'nt just die from a counterattack.

She can probably work still. Her speed isn't gonna be great where she can quad-attack, so you want to focus on her damage she can put out with Amiti, but you already have Death Blow. I wouldn't consider going for Life & Death because her Resistance will be above average and allows her to take a hit from common green mages like Cecilia and Nino (Flier Buffs to raise her Res to 37).

I have a -def +res Elincia, and kept all of her base kit and added Fortify Fliers, Galeforce/Moonbow with Heavy Blade (Sacred Seal). Galeforce + Heavy Blade with Flier Buffs can be a monster on Chain Challenges/Tempest Trials, but if you're going for Arena uses, Moonbow + Heavy Blade and Flier Buffs allows her to get a Moonbow Proc provided the enemy has less than her attack (57 with Death Blow and Flier buffs). I HIGHLY recommend her to be on a Flier team. If anything else, she will be use to use her Flier Formation to get your units out of sticky situation, or into better position. She's just a good unit to have on the team.

If she won't be running on a Flier Emblem, again, max her damage output she can do with her legendary weapon. You can throw on Threaten Defense on her, or Defense Ploy because of her above average Resistance stat. A seal that you can use are Heavy Blade or Quickened Pulse with a short cooldown Special like Moonbow (Quickened Pulse is reliable for the ensured proc, but Heavy Blade if she will be the main damage dealer or highest damage dealer).

Also, rarely would my Elincia need to Quad-attack to finish an enemy off, so just make sure she has her buffs and special procs and she will be an amazing unit to have.