Lyn

-atk? that's really not good. lyn is really bad if she get -atk ivs. you want to have +atk or neural to at least perform better.

im guessing its the normal Lyn. i wouldnt use her as a atk bane is her superbane. but if u really wanted to make her work u can try the vantage/desperation/brash assault/quick riposte build with fury. costs alot and needs to hit certain hp point to go crazy but if u like lyn that much. or u can keep her a slot to try to patch her atk up by playing a vantage/ desperation build. with this she has to get lower then 50% to make her damage roar. under50 she can retaliate melee units on enemy phase and blaze thru with player phase( assuming she doubles her enemy) i would get her effect weapon upgrade

I assume you man regular sword Lyn. And like deadly says -ATK is very painful. It is a superbane AND her attack already starts sub 30 to begin with. As for what to do. You have Galeforce fodder now. If any of your units want galeforce now you don't need to spend 20k feathers for it.

If you don't want to use her as fodder, you might attempt to patch up her offense using Triangle Adept or Death Blow in the A-slot, and an Atk Seal. If you build her, you'll hopefully pull a better version later that you can merge into.