Better as fodder?

Depends how much you want a Legendary Eirika, because who knows when you'll get her again unless you have the cash. Eirika's strength relies on her B skill special to deal 50% of the enemies DEF every special not to mention she has moonbow. Her weapon also grants her +3 Atk, so that it keeps it from being bane. Her A Skill is goddessly so no one would blame you if you gave to another.

Give her moonbow or glimmer and keep lunar brace. Then when her special triggers, her Atk literally makes zero difference.

Try buffing with hone cavalry and see how you like the base kit