|IVs: Both +Atk and +Spd work wonderfully on Lewyn. +Atk boosts his base attack to 37, and +Spd boosts his already impressive speed to 40. His bulk doesn't matter all that much with this build, but -Res is picked because -Def and -HP are both superbanes, and decrease his score by 4 instead of 3, which is bad for arena scoring and all that.
Weapon: Forseti is obviously Lewyn's weapon of choice, since it's his prf, gives him more speed and is basically the reverse of Desperation 2 (≥50% as opposed to ≤50%).
Assist: Draw Back is the assist Lewyn works best with, since it lets him reposition units without putting himself in the front lines and risking him taking damage from some melee unit.
Special: Glimmer works best for this build, as Lewyn will activate Glimmer in pretty much every fight unless he dies, and it amplifies his already great damage.
A Passive: Life and Death 3 is optimal for Lewyn. With his weapon's Desperation being backwards, he doesn't need to take damage for it to kick in, and as such his defenses shouldn't be that much of a big deal. If you're worried about Lewyn's defenses being that low, Swift Sparrow 2 is always a good alternative, especially since he already comes with it.
B Passive: Special Spiral 3 is a perfect skill for Lewyn; the early double he gets from Forseti, paired with the Flashing Blade seal, makes Lewyn activate Glimmer pretty much every fight.
C Passive: Odd Atk Wave 3 is optimal for Lewyn's C slot, since it increases his already amazing attack every other turn, and he comes with it by default.
Sacred Seal: Flashing Blade 3 is the best seal for Lewyn, as it makes him activate Glimmer even more than he already would otherwise; with his speed at 45 (48 with +Spd) from the LnD + Forseti buffs, there are very, very few units in the game that can cause him to miss the speed check for Flashing Blade, and thus Lewyn gets another -1 on his cooldown pretty much every time he hits.