L'Arachel

Keeping Res Tactic is a good idea, I think either Atk Res 1 (Which you get from a four star TailTiu) or maybe Death or Darting Blow could be good as you can easily get those skills and either keep her B slot or swap it for either a seal skill or red/blue tomebreaker. For the seal I am unsure since I don't know all the seals you have, maybe a fortify or Phantom Spd?

I think the following will work fairly well for her: Ivaldi, Reposition/Draw Back, Iceberg/Moonbow, Fury/Darting Blow, her native Renewal or Vantage or Desperation if she gets Hone or Spur Speed and keep Res Tactic.

This is a good budget set for her and def bane would be better than a res bane

Coincidentally, my L'Arachel, a +1 merge, has the same spread. Thankfully I am going the right direction with her judging by the other replies (kept tome and passives, added Darting Blow). However, I gave mine Glimmer, though Iceberg is also a good option... Got a spare Niles that I can sac if needed. :p

Darting Blow. Is the best option. I use mine with Darting blow, she has her Attack really high but her speed fails a bit.
With her tome, Darting Blow and Desperation, she becomes a MONSTER. She is really underrated. I also use Fobos.

I use her with my cavalry team and debuff team (Gunnthra, Catria, SM Eirika, Priscilla, Elise, ...).