tfw you can't decide which refining options to prioritize...

I have a +Def/-Spd Monster Jakob who has already been invested with a Brave Bow+ build and his budget build. I managed to pull another Leon in hopes for inheriting Slaying Bow+ and the main question is...

Through refining, what would be the best thing for Slaying Bow+ for Monster Jakob? +Atk or +Def? Make him hurt more or make his Bonfire damage hurt more?