Felicia Build

Give her Glacies and then merge her into one that isn't -res. It's one of her important stats and lowering it makes her directly worse, because she needs all the res she can get, since she can't abuse the weapon triangle when dealing with mages.

I recommend an AOE special, prefferably the "Blazing" version.
She can use it to good use since with her Adaptive damage she can damage every unit in the game to some extent and in the battle field if it is possible she can damage every enemy unit in the field making it easier for your team(exept when there is vantage or wings of mercy).

But if you don't like the Idea then since she is a 3-4*, give her glacies and merge that current Felicia to a non-res felicia when you get one.

You don't actually miss out on that much damage from the glacies. I have Felicia+6 with neutral IVs, and pretty much everything dies on the glacies no matter what except sometimes raven tomes or very strong mixed bulk, but she usually falls short by more than 2-3 points from the lost res.