-Atk +Spd Leif

Yes, that is still salvageable. Give him the basic Fury + Desperation + Moonbow/Luna + Atk or Spd seal and he's good to go. Don't forget his Light Brand gives him +7 damage on high def low res enemies anyway.

+Spd Leif is a monster. If you want to highly invest in him you could run Slaying Edge with Moonbow - the constant special proc's can make up for the Atk bane. Or just run Light Brand and make sure you're targeting high Def enemies. At worst, I'm sure Light Brand will get a refinement sometime in the future so at least hold out until that.