Fjorm

Neutral nature Fjorm is actually a good offensive Arena Unit. Why?
Brave Bow+ Ensures double attacks to KO quickly.
Death Blow 3 gives Fjorm +6 Atk when initiating and Quick Riposte guarantees 4 strikes with Brave Bow+. At a base 30 Atk, with a weapon equipped, this means that, when initiating combat, Fjorm has an Atk stat of 47, which is rather good for a Brave Bow User.
Fjorm also has good defenses, allowing her to be effective on both phases, rather than units like Reindhart, who are effective only during the player phase.
It's also worth noting that this Fjorm set gets a high arena score due to skill inheritance.

A powerful 42/42 defense stat spread when attacked at any range, as well as defense ploy to make up for her less than stellar attack stat. just put her in the frontlines and laugh.

This Fjorm set is more offensive taking advantage of her massive 46 base ATK.

Keep Leiptr as her weapon, and support skill is up to you. Instead of her signature “Ice Mirror”, equip Iceberg to take advantage of her 34 base RES.

Her A and B skill slots are the basic Fury/Desperation combo. C slot is up to you of course!

Again, use Distant Def/Deflect to keep her tankiness, or Savage Blow for some extra damage.

Fjorm’s base stats are really good, that she may be a very versatile unit! This set is pretty budget, since the only skill you need to inherit is a support skill (And Fury).

Let’s keep her legendary weapon so we don’t have to use our A slot for Distant Counter, and instead have something else.

For her support, it could be anything so Reposition, Pivot or maybe a Rally!
As for a special, Ice Mirror would be great espcially since it’s her default.

Since it’s budget you can keep her default kit, or change her C and/or B slot skills. It just depends on your personal preference and team. However, for even more power, go with Fury 3 as her A slot (or LoD but Fury is a little easier to get at level 3).

As for her Sacred Seal, go for Distant Defense to further tank hits from afar, especially bows and daggers.

Leiptr's an alright weapon, but I was wondering if a build with Distant Counter in the A slot might open up a superior Fjorm (at least for filling in the distant counter lance infantry niche). The above build is probably not optimal, but I'm hoping to open up conversation about options outside Leiptr now that weapon forging is a thing.

The idea for this particular build is to enable one hundred percent uptime on Fjorm's special (so long as she can counter/attack). This build will work best if paired with a character that can heal, since Fjorm's special requires baiting and, even with its significant survivability increase, baiting more than one attack in a match is hard to pull off without healing.

This build sacrifices some significant stats from running Fury 3 in order to maximize Fjorm's gimmick, but I think the tradeoff in survivability when fighting ranged units could be worth it for some teams (also--fun!). A doubled Fjorm will have her shield pulsed Ice Mirror go off twice against a ranged unit she can counter. Obviously, though, this version of Fjorm will end up being more vulnerable to close combat units--especially green physicals like Hector. The +HP and speed from the lance is pretty solid, but, without Fury, Fjorm sacrifices an additional 3 defense.

After doing some calcs (warning: am bad at calcs), I believe this Fjorm is suitable for countering most unmerged mages outside Reinhardt. She seems to live /kill a perfect IV L&D Nino w/ Hone Speed/Attack and spur speed. She is suboptimal against Reinhardt and most bow units, however, as slaying lance does nothing particularly special for her against brave and sweep weapons. Be careful of baiting strong melee units and units in desperation range, too, as they can often kill her. If you need a character to counter multiple nonconsecutive attacks from strong mages or to survive against bulky mages (should the meta shift), this Fjorm is quite capable, but if you want a more generalized Fjorm then fury in her A slot is likely superior due to the added bulk and attack.