Best B Skill for my La Lyn

What weapon does your Lyn use?

With +spd on LA Lyn I recommend special fighter easily. Blast everyone to death with the glacies baby

I haven't thought too much about Niles yet, but I've figured out two builds I think are good for him.
1. Fishie bow special fighter moonbow (bold fighter would be ok too but I think it's a waste when he's already so fast and comes with swift sparrow)
2. Shining bow special fighter glacies with brazen def/res (sounds almost like something on a restaurant menu...). Refine the bow for def if he doesn't come +def. It'll need ward armour support, but he can probably get his defenses up to the mid 40s which will probably suffice for taking an extra hit or two.

do you mean special fighter or spiral ?

if its special fighter, that would be the best for her.
atk/spd bond, blarowl(spd) and spd+3 seal, she will have 51 speed with 1 allies (most likely her armor march friend)

for Niles, I'd give him firesweep bow, moonbow, special fighter, and armored boots. so he can be an only armor in your team, to deal damage for free, with better movement than infantry archer.

Both units are very versatile. Niles can wield a pseudo-Reinhardt set (without the range, of course) without setup with Firesweep and Bold Fighter, while Lyn requires setup for Juicy Wave. But she can wield Blarblade and Blarowl, two very good weapons.

Niles has 164 bst, so needs a superboon (in either HP or Def) to equal Lyn's 165 bst and bin. If we got a blue armor duel skill (which we very well may not ever get, the duel skills can only boost the older or ranged armored units, and at most only 2 bins), then I suppose Niles has slightly more attack or speed potential at max arena score discounting an owltome on Lyn (which I use).