best boon/bane for cecilia?

+ atk - def is her best boon/ bane for the blade set. Speed is not that useful because she will one shot almost anyone anyways.

I'd say +Atk, she's not doubling much even with Goad and Hone. So I'd pick +Atk / -Def

For the Bane though, I personally can't decide between -Def or -Res. She loses 4 in Res but this way she can use Raven!TA better, while with Def, although she still lives a Brave Lyn, she'll take more damage from tanking her and Brave Bows users.

+atk is great for her. -any of the others, preferably not res as she needs that for mages, especially Reinhardts if you're going with the blade tome build instead of Raven tome TA build.

For the blade build, +Atk - Def is probably her best IV Set.
She won't be doubling much anyway, but no one (except red TA) will survive her fortified & honed shot.

Are you sure Darting Blow is your best option there ? Fury / L&D are great on her.

For raven tome, -def is a bit more punishing. Neutral is better maybe.

If you only want to build one Cecilia, I'd still squeeze the +spd one. And I guess Blade is so much more powerful with +Atk that you can sacrifice neutral and SOME efficiency as a raven :)

In my opinion...
For Blade, as others have said +ATK is the best.
For Raven, I'd go with Neutral due to -DEF being troublesome. Especially with Brave Lyn riding around ignoring her buffs.

I've had the chance to run both +Atk and +Spd Cecilia with bladetome in a horse emblem team. My conclusion was that +Spd is significantly better in the long run, especially for TT/CC content (inflated stats).

+Spd Cecilia with Hone cav+Darting blow+Spd seal reaches 42 spd, which is enough to double quite often. Slap desperation in her B slot and she'll become a goddess.