Which Nowi to use?

Definitely build on that +ATK -hp and then just merge the rest into her. +Def is probably her best boon unless you're going to heavily invest in her SPD (mine is +SPD -res as seen in photo) which can definitely be worth it to avoid doubles.

Best A skills for her are Fury 3 or Steady Breath. B skill would be vantage or quick riposte probably. C skill is whatever your heart desires.

if it's for arena and you plan on merging, +def/-spd

at high merges/arena you won't prevent doubles from the big damaging threats even with +spd. with more def, you can greatly reduce and in most cases nullify any red/blue non-special damage.