How to optimize my Ryoma? (-Atk/+Def)

So, I know Fates isn't the favorite around here,but of all the Fates characters, Rockin' Ryoma is by far my favorite. He was a one man army(not kidding). However, here in Heroes, I didn't get that lucky. He got cursed with one of his worst banes: atk, but got at least a passable boon: def. I pulled 2, and I have no idea what the IVs on the others were(I pulled them and merged them before I was aware of the IVs system) He's inherited some decent skills. Fury, Darting Blow, Vantage, Desperation, Threaten Speed, Luna. I'm not opposed to using some high level inheritance to fix him up if necessary.