Nowi optimal IV

I would personally pick +Atk / -HP as a general choice. HP is a better bane than Res because Nowi has a high chance of being doubled, which means keeping her Defences intact is more important than HP in my opinion.

+Spd Nowi is also a good choice but -Def isn’t as desirable as HP or Res, but it’s still a good choice if you want the speed, and keeping her Res intact lets her deal with mages better anyways.

+Atk / -Res and +Def / -Spd would be great if you decide to run Steady Breath instead of TA or Fury since the former allows Aether to do more damage and still keeps her physical bulk intact, while the latter gives her more physical bulk to synergise with Steady Breath.

In summary, +Atk/-HP is the most flexible, +Spd/-Def if you want to take the speedy route and help her avoid doubles, +Atk/-Res if you don’t need her to deal with mages or if you’re deciding on Steady Breath, and finally +Def/-Spd if you’re going for a physical tank route.

You generally pick out her IVs depending on what YOU need her to do, so the major thing to keep in mind when deciding is to first pick whether you prefer Speed or Attack by deciding which one suits your playstyle or the role you want her to fill better, then it’s just down to deciding which bane hinders her performance in that role the least.