Breathbreaker, while unlikely and rather useless for Lucina, would benefit Chrom. Still, his Res is so shit he'd likely need a Brave effect instead to actually benefit from it.
With Lucina's rather low defenses, but lower Res, she could benefit from a "dragons now target Def" effect, which would be even more beneficial to Chrom.
Or a "reduce damage from first hit from dragons by 50%" thing.
I'm assuming the only real benefit Lucina would really get is Renewal 3, or 4 if you want to push it.
It certainly wouldn't save her from being a second-rate sword, but at least it would help her support role.