Brave Lucina IVs

Who do you want her to fight and what values of spd and def do they have?

+Def -HP easy! I find an HP Bane is typically the easiest to alleviate (in this case a blessing and L!Hero will fix it right up without any skills/etc.).

If you want to use her as a support/tank hybrid unit, +def -hp is *great*. She can tank quite a few things with a +def IV, and and if you put Renewal in her B slot she'll have great sustain as well. (It also helps mitigate the hp loss from Fury if you decide to put that in her A slot.) My first Brave Lucina was +def -hp and she did a lot of great work for me, especially during TTs.

If you want to use her more offensively, you may prefer +spd -atk, but offensive Brave Lucina is more expensive to build, and being -atk will likely hurt her in this regard compared to some other bane, but you can probably still work with it if this is your inclination. This IV set might also work for a pure support build if you aren't worried about using her in combat and just want her to avoid as many doubles as possible, although in that case I personally think that making her tankier with +def is more useful and more fun.