Best build for a IV neutral Fjorm?

Her default kit(Atk/Def Bond-Shield Pulse-Drive Atk) is fine, and it got a pretty big buff with the addition of Quick Riposte seal. This way she can double while still being able to run Shield Pulse so she can use Ice Mirror.

If you want some other options, you can go for Fury or Steady Breath(if you have a spare Brave Ike)/Quick Riposte/Flexible C skill with Distant Defense/Deflect skill. Special for this build would be Iceberg, since you won't have Shield Pulse. Ice Mirror can work as well, but Iceberg is usually better since with Ice Mirror you don't have the same synergy without Shield Pulse.

Her base kit is a fine build, especially with the Quick Riposte Seal. I have her paired up with a unit that has Rally Speed/Defense, Spur Speed/Defense, and used to use Spur Defense Seal until Heavy Blade Seal became a thing.

Still, that is +7 Attack, +8 Speed, +13 Defense, +2 Resistance. She can basically punish anything that attacks her that isn't Brave Lyn.

Because of how her A Skill and Ally Support work, having her paired up with a unit using a Rally and Spur just makes so much since.