Lewyn's best IV's

-res is the better bane, but it's not enougth as a reason to take +atk, his spd stat is at a point that he benefits more from +spd rather than a +atk.
If you plan on using him in a team that can keep his spd constatly buffed then go for +atk -res

+Spd is better. Spd helps him with Forseti's effect, and Def is the ideal dump stat over Res.

Since Lewyn is a ranged attacker you probably be fighting mage or archer the most. but since he dies to physical attack anw, just let him be a little better at taking magic attack and can avoid doubled?

I don't think he really cares about either banes, since they're his best "dump" stats and he's awful at them, either way.

On the other hand, between +Atk & +Spd is apparently debatable. I thought +Spd was an obvious, but it's not like he isn't doubling even without it (neutral Lewyn with L&D reaches 45 Spd). It's not like the extra +3 Spd is doubling stuff like Ayra and Karla, but he's doubling pretty much everything else and +Atk gives him far stronger AoEs.

It's kind of just up to personal preference, really.

Im kinda shock at the lack of people telling you +spd is redundant.

You have 37 speed base with a prf that gives +3 speed. With swift sparrow added you go up to 44. With a buff to your speed you get 50 speed. If you can appropriately buff his speed you double everyone with 45 speed or less. More speed at that point is redundant and increasing with ability to kill slower targets with more attack is far more valuable than trying to double the LnD Karla