+Spd/-Def or +Atk/-Res Elincia?

+atk is preferred to synergize with her bravesque weapon, unless you want to do some quad shenanigans, which most of the time end up being a gimmick and nothing more.

I would go with the +Atk one. Elincia's speed is good enough to double those who are bulky enough to survive her first two hits, generally speaking at least. Since you have some merges on her, the speed she gets through this and probably flyer buffs is more than enough to not get doubled herself and secure most kills. Atk in general is her preferred boon I would say, I have one myself and she performs great. Though if you ever happen to get an Elincia that's not -Res but -hp I would replace that, since with normal res she can at least take a hit from some mages or dragons, the boon is more important though.