Best IVs and build for Leon

+atk Leon needs Death blow and Brave bow+ to be effective. There is a way more expensive build that I am using with Close counter and Quick riposte on a +def Leon and +def refined Slaying Bow. It depends on what kind of a unit you want to make him. Brave bow for player phase and Quick riposte or a breaker skill and Slaying bow for enemy phase. Also, best IVs are either +Atk or +def and -res.

+ att -res.
Weapon:BB+
Slot 1:Death blow
S2:Cancel Affinity
S3:Threaten defense

I run two builds on my +Spd-Res Leon:
Slaying Bow+Spd, Bonfire, Close Counter, Vantage/QR3/(or Bow Breaker if I need another Lyn counter), Threaten Attack (+QP sometimes)
or
BraveBow+, Bonfire/Moonbow, DB3, Vantage, threaten attack.

He's honestly viable either way, it just depends on who's supporting him in the team at any given time.