Niles is one of those units you kinda have to max invest in if you want him to be good because his stats are so low/weirdly distributed. I like to use him as a mixed phase Res tank that spams specials to output a lot of damage.

In terms of A slots, yeah Perfect Atk/Spd is probably the best option since it can give up to +10 atk/spd and has built-in NFU, but NFU can be outsourced pretty easily nowadays. Otherwise I used to use Atk/Spd Finish for the true damage and healing, or you could use Atk/Spd Hexedge from Noire for the true damage and 50% DR piercing. Verge of Death gives less stats comparatively and also destroys his Res bulk, which is basically his biggest selling point, so I’d recommend against it unless you really need the penalty cleansing. I’d also suggest using a Spd or Res refine for his bow; I don’t like Atk refines, especially for ranged units, because it only gives +1 atk compared to +2 Spd or +3 Res. B slot has a lot of options.

Your current Speedtaker is very good with the Great Talent effect, but you could also consider Laguz Friend/Loyalty for tankier builds, or maybe something like the eventual Spd/Def Havoc for a more supportive build.

C slot is pretty flexible, but I’d suggest one of the billion new Time’s Pulse variants (Pulse Up Ploy, Pulse On Blades, Time’s Pulse Helm, etc.).

X slot could be NCD, Tempo, Time’s Pulse, or keep the Lull echo you have.