+Atk/-Spd or +Def/-Atk Lewyn ?

I’d say +Def -Atk. Sure, the other Iv set has +attack, but Forseti’s effect only works if you guarantee that he’s gonna double, so you really don’t wanna sacrifice speed.

He also comes with Odd Attack Wave, which helps his attack, and Special Spiral should provide another source of damage provided he doubles.

You could try replacing Swift Sparrow and go for Life and Death instead for that extra point of attack(and speed), he shouldn’t be taking hits anyway so the defense reduction pretty much has no effect.

Usually Lewyn is built with a 4 cd special and Flashing Blade(or Heavy Blade) thanks to Special Spiral’s cooldown reduction abilities. Instead you could just leave Glimmer to help the -atk for the extra damage in his first initiation to secure some extra kills since with Astra/Dragon Fang he’s not gonna proc a special the first round.

Don’t worry that much about atk bane, he’s still a magic damage dealer in a game where a lot of characters have low resistance. Just try to buff him as much as possible.