I've always lamented how Niles got really shafted with his stats in this game, especially since I consider him one of the best units in Fates. So I was playing around to see what I could do with him, and I've hit on something much better than I expected. Niles can actually be a decent offensive archer with this build. He's not gonna replace Bridal Cordelia or Brave Lyn anytime soon, but he should be totally useable like this.
IVs: +spd -hp (or -def)
Weapon: Slaying Bow (+spd)
Assist: Ardent Sacrifice or any
Special: Iceberg
A: Brazen Atk/Spd
B: Desperation
C: Def Ploy
Seal: Flashing Blade
Stats at 5*+0: 36/37/39/17/34
Stats at 5*+10: 40/41/43/21/38
(Good lord, the poor guy has more speed than attack even with his weapon equipped, wtf IS.)
From the beginning, Niles was clearly designed to be a low-attack res tank who did most of his damage with his special, and I actually got a bit of use out of him this way in my early days of playing the game. (Then I pulled a Klein, and poor Niles was lost to the abyss.) But with the flashing blade seal, Niles can now do damage with his special consistently. Assuming he doubles his opponent, he can proc his 2-cooldown iceberg every player phase, for a minimum of 17 extra damage. And once he's in Brazen range (which he'd want to be anyway so desperation is active), his attack and speed would reach 44/46, with his attack going up to an effective 49 if he can land def ploy (that's starting to look almost convincing). These numbers are assuming no merges - with merges he can get quite a bit higher.
I love this build especially because Niles is one of the best users of it. Yes, +res Summer Gaius or +spd Innes could theoretically run it a touch better, as they have the same speed and either 5 or 8 more attack, which would make up for the slightly decreased iceberg damage and less consistent ploying since they have 3 less res. But Niles, unlike the other two, is *much* more available for merging and a lot easier to obtain with the right IVs. I know, even with this build no one's gonna be reaching for Niles over their Brave Lyns, but for those of us who like Niles...
Note on IVs: the +spd boon is mandatory, but the bane can be either def or hp. Go for def if you care about preserving every last bit of his magical bulk. Go for hp if you care about bst (def is a superbane) or if you want to still be able to use ardent sacrifice to trigger desperation even at high merge levels.
Note on weapon forge: I considered a +res forge, for a higher-damage iceberg and better ploying, but ultimately I went with +spd, because if Niles can't get the double he's sunk, plus most units with more than 33 res have low enough def that it shouldn't be a big deal. (Outside of some armors, Fjorm, and a couple of dragon builds, anyway, but those units are likely to give Niles a bad time regardless.) A +res forge might become more viable if Niles has increased his speed through merge levels, but even then you may choose to run more speed.
Note full of wishful thinking: Hey IS, want to give Niles a prf? Maybe with a Wrath effect? I know, Wrath isn't normally available for ranged units, but flashing blade isn't available to fliers and that didn't stop you from putting it on Caeda's weapon. Niles has the same attack as Caeda. Like her, he needs something major to make him viable. Just saying. ;)