Skills for Corrin (M)

I've been working with this bad boy for quite some time before i kind of left him to gather some dust on the bench. Today tho i summoned another one that i was able to upgrade to 5* and use it for merging. Now, there are things i'm not totally sure of. Namely his Special and his B slot skill. He's +Atk -Res so he's a heavy hitter. You guys have any idea on what i could give him? For his B slot i was thinking on Desperation, but Shanna just won't appear. Also i've heard from other that +Spd i better than +Atk but i'm not entirely sure