Got 3 Nowis (+hp - atk), (+res -def) and (+hp - def)

All those ivs are meh, but out of them the +hp,-def is the best one. This keeps her defense intact with the cancellation of stats on that end and increases her res tanking with more hp. This also allows her to run panic ploy and infantry pulse better. Nowi is one of the best in the game right now, even with bad ivs. It is worth it as she can usually take down Falchion users and only has to fear Naga/Divine Naga, and some greens(she does tank some greens and kills them back, unless you are running Triangle Adept).

There are several different builds that will have different ivs for each. You have the standard enemy phase build which is a multipurpose tank with self sustain in having aether almost every combat phase or doing masive damage with accelerated special

Lightning Breath+(+def or +res if you want her to tank mages/dragons better)
Reposition/Swap(dual rally for arena scoring)
Aether(Ignis/Bonfire for a budget build if going +def refinement, Glacies/Iceberg for budget build going with +res)
Steady Breath(Fury for budget)
Quick Riposte
Panic Ploy/Infantry Pulse/Threaten Res/C skill of your choice
SS: Distant Defense/Close Defense/Panic Ploy/Deflect Magic(mainly for Rein/Olwen)/Atk Smoke
iv:+atk/-spd, +def/-spd, +res/-spd(to help with mages and other dragons if you need it for your team) Those are the optimal ones for that build but almost any iv will work. I myself run it with +spd/-hp to avoid doubles and not fire off an aether at an almost dead enemy.

Red Slayer Build
This set mainly focuses on taking out anything red

Lightning Breath(+def or +res if you want her to tank mages/dragons better)
Repostion/Swap(dual rally for scoring)
Bonfire/Ignis/Iceberg/Glacies(Bonfire/Ignis for +def refinement, Iceberg/Glacies for +res refinement)
Triangle Adept/Fury
Quick Riposte/Swordbreaker
Panic Ploy/Infantry Pulse/Threaten Res/C skill of your choice
SS: Distant Defense/Close Defense/Panic Ploy/Deflect Magic(mainly for Rein/Olwen)/Atk Smoke
iv:+spd,-res/+spd,-hp Those are the optimal ones, but any iv can work.

Those are the main two builds for Nowi. You also have some niche builds with light breath/dark breath, but those require +spd, windsweep, and distant counter available. I can post it if you want.