Should I Sacrifice a Nephenee +atk -res to inherit Wrath to another unit?

Neph is a solid lancer. The real question is: do you have enough good blue units for Arena Assault? If so, then I wouldn't really develop Neph, and instead use her as Wrath fodder(LA!Hector, BK, etc.).

If you don't have many lance/blue units, then she's a solid addition to Arena Assault that doesn't really require that much investment. However, she's not top tier.

I would keep her, those are pretty much perfect IVs.

I like +spd on Neph, but +atk is very solid too. I'd keep her.
The only thing that could potentially make me sack mine is a perfect IV Ayra.

Definitely keep her. She's one o the best infantry lance users for me, give her +Spd refine on her lance and she'll be very difficult to double while having good defense and attack with Wrath boosted Bonfire/Moonbow.