Just build her as support if you don't want to sac her. I'd keep grimoire and give her iceberg. I wouldnt give her a fury but if you have plenty-why not? You can give her darting blow 3 for A. On b slot, just, it can be anything you want. Hone flier for c skill definitely. She's mostly there to buff units.
What to do
So today, I pull a Halloween Nowi. I was super excited at first than I realized it was -spd +hp. I don't know what builds should I run and I'm free to play.
Well considering the fact that Nowi's speed isn't THAT amazing to begin with, -Speed isn't as terrible as it would be on a unit such as Cordelia or Summer Corrin. You have a couple options. You could make her a Nuke with a blade tome and Deathblow, you could make her a Brash Desperation unit with Fury and either Grimore or a Blade tome, or you could make her a warping Res tank with Escape route and Attack/Res bond. Nowi is a pretty alright unit with a lot of different build options, so don't worry too much about.
To be frank, either you get a better IV before the end of the banner or you use her as a fodder for Atk/Res link, otherwise she will just be a League B unit.
She desperatly needs +Spd to be used at her top level but she is a good unit regardless of the IVs as long as it's not -Spd so go for broke, get another one :p