Nephenee's Slaying Lance+ upgrade

I'd suggest just going for +atk

My Nephenee is +Attack and i use her base kit w/ fury and she tends to OHKO a lot of characters she initiates on and her decent def allows her to bait a lot of phys units and get off a strong moonbow even when she is in wrath range.

I imagine you can emulate the atk boon with deathblow instead of fury but that sacrifices her decent enemy phase capabilities somewhat.

Otherwise, go +spd if you want to proc moonbow twice in one battle (with Fury or Darting Blow)

Another good option is going +Def and swap moonbow out for ignis and give her armored blow and the brash assault SS so she can activate a wrath boosted ignis every match while taking little to no damage

Speed. She reaches a 50/40 Spread with your IV's, base kit and no seal, which allows her to avoid most doubles of unmerged units

I've got +Spd and with Att/Spd 2 (for Tempest and Chain Battles) or Fury 3 she has 40/41 speed so I went with might. Lacking a +Spd IV, I'd suggest speed as reaching 40 prevents almost all from doubling you which is better for surviving than 4 extra defense.