Ninja Laegjarn

Weapon: Flamefrost Bow

Assist: Ardent Sacrifice / Flexible

Special: Moonbow / Iceberg / Deadeye

Passive A: Close Salvo / Close Counter / Close Reversal

Passive B: Vantage 

Passive C: Atk/Def Rein or Hold / Joint Drive Atk / Atk/Res Oath 4

Sacred Seal: Atk/Def Catch / Heavy Blade / Fierce Stance

• If a unit has a dual-phase brave weapon, then an all-countering Vantage build recommendation is pretty much obligatory. Due to the sheer amount of damage from double strikes, this gives you a potential checkmate option after countermeasures are taken care of. In Laegjarn/Fjorm's case, Flamefrost Bow offers exactly that and can provide additional striking strength through its ally/enemy bonus-based stat swings — like Laslow before them, take care to specifically provide stat buffs throughout the team to get the most efficient use of the weapon's effects. 
• Flamefrost Bow also significantly improves Laegjarn/Fjorm's defenses given the right teammate buffing setup, which allows them to safely whittle themselves down to Vantage's HP requirement. 
  • For a safer method (given that optimal setup matchups won't always be present, especially in Summoner Duels), Ardent Sacrifice with Winter Bernie or Brave Lysithea's HP-lowering properties is a good out-of-combat option.
• As usual, Vantage sweepers want to maximize the number of enemies they can counterattack against possible to avoid being vulnerable to kills. Out of all the Close Counter-based options available, Close Salvo's Atk increase and self-recoil make it the best choice. Do not settle for Foils as maximizing counterattacks, again, is the goal here.
• To finish off the build and improve the build's killing strength, Laegjarn/Fjorm will want to dedicate the C and Sacred Seal to reliable damage-improving passives. 
  • In the C-slot, Atk/Def Rein (or Hold if you're an extraneous spender), for example, does exactly this while also further improving Laegjarn/Fjorm's bulk. Likewise, Joint Drive Atk benefits other allies with easier positioning, and Atk/Res Oath gives additional visible bonuses and also more mobility options, and both encourage you to keep up Flamefrost Bow's ally nearness requirement at any rate.
  • A basic Atk Sacred Seal like Atk/Def Catch or Fierce Stance should easily do the trick. To potentially trigger Iceberg or Deadeye from full cooldown on every counterattack, however, you can run Heavy Blade to accelerate Special charging and accommodate this with significant Atk support from allies.

Weapon: Flamefrost Bow

Assist: Positional Assist 

Special: Deadeye / Glimmer / Moonbow

Passive A: Atk/Res Catch 4

Passive B: Seal Def 4 / A/D Far Trace / Wings of Mercy

Passive C: Atk/Def Menace / Atk/Def Rein / Atk/Res Oath 4 / Joint Drive Atk

Sacred Seal: Atk/Def Catch / Atk/Def Solo / Death Blow / Heavy Blade

• Laegjarn/Fjorm's base build is a lot more than serviceable for the most part; it provides almost everything they need for a complete stereotypical Brave Bow nuke build, in fact. With the extremely high damage swings from Atk/Res Catch, Seal Def 4, and Flamefrost Bow, Laegjarn/Fjorm needs little more than a few slight kit optimizations. 
• As Atk Smoke 4 is a more enemy phase-oriented that requires Laegjarn/Fjorm to participate in a fight before kicking in, more immediate C-slot skills with less setup are sought out. 
  • Atk/Def Menace is one of the best options out there as is directly impacts the enemy's Defense while improving Laegjarn's own Atk, giving the largest damage differential of other skills. 
  • Else, Atk/Def Rein, Atk/Res Oath, and Joint Drive Atk all work as great in-combat options. 
• For alternative B slots, A/D Far Trace is worth considering over Seal Def 4 despite its smaller in-combat debuff; the post-battle mobility offered by its Canto effect gives Laegjarn/Fjorm a possible avenue of escape after completing a match. Likewise, Wings of Mercy grants significant mobility assuming a greatly damaged ally is in play; Laegjarm/Fjorm can thus lend their strengths over on AR-O Galeforce teams as a technical option (due to their self-refresh).
• To finish off the build and improve the build's killing strength, finish off with a damage-increasing Sacred Seal. A basic Atk Sacred Seal like Atk/Def Catch, Death Blow, or Atk/Def Solo should easily do the trick. To potentially trigger Deadeye from full cooldown on every initiated attack, however, you can run Heavy Blade to accelerate Special charging.

Weapon: Flamefrost Bow

Assist: Positional Assist 

Special: Bonfire / Ignis 

Passive A: Close Reversal / Atk/Res Catch / Close Foil / Close Counter / Close Salvo 

Passive B: A/D Far Trace / Mystic Boost 4 / Seal Def 4 / Quick Riposte

Passive C: Atk Smoke 4 / Atk/Def Rein or Hold / Atk/Def Menace / Def/Res Menace

Sacred Seal: Atk/Def Catch / Atk/Def Form / Atk/Res Form / Iote's Shield / Heavy Blade

• This build is somewhat to the previous Vantage build, but focuses instead on making use of the combined tanky might of Laegjarn/Fjorm's statline and Flamefrost Bow's potentially high defensive bonuses. Basically, we intend on taking heavy hits and retaliating instead of sweep — and a lot of power is still maintained with Frostflame Bow's double strikes and additional damage passives. 
• A Close Counter-based A-slot is maintained as a catch all-option and generally making Laegjarn/Fjorm a more independent unit overall. Though, given Near Save support, wanting to keep matchups strictly ranged, or wanting to maintain killing power in both phases, their default Atk/Res Catch may be a preferred option as well.
• One significant alteration from the Vantage build is, of course, the lack of Vantage in favor of more sustainability or tank-focused skills. 
  • Mystic Boost 4 is one of the best options here, with a pack of effects that suit this build perfectly: lowering enemy damage by -5, healing a fair amount of HP back after every match, and the ability to pretty much decimate any Dazzling Staff healers that come Laegjarn/Fjorm's way.  
  • A/D Far Trace provides a form of Canto, something that Laegjarn/Fjorm otherwise oddly lacks for a 2022+ premium flier. The added escapability movement after battle is always nice, but the other main draw is the combat debuff of Atk/Def -3 which consistently applies in both phases. 
  • Seal Def 4 is a pretty good option to keep; it doesn't improve bulk or sustainability, but instead increases damage while offering a form of Special denial (enemy CD +1) if a penalty is present on the enemy.
  • Quick Riposte is a bit on the lower grade scale, but the additional follow-up may allow Laegjarn/Fjorm to confirm a kill with a total of four hits dealt in retaliation. 
• In the C-slot, Laegjarn/Fjorm's default Atk Smoke 4 takes center stage here as it significantly improves matchups with follow-up denial as well as mass Atk manipulation. It requires one combat to completely set up though, which isn't always optimal; this is where Atk/Def Rein or an appropriate Menace comes in as more immediate alternatives at the cost of the follow-up blockage.  
• Atk-boosting passives are preferred in the Sacred Seal to make up for Frostflame Bow's low Mt, and given the build's enemy phase nature and inclination to be near other allies, Atk-based Forms tend to offer the most gains. Heavy Blade also appears again as a way to accelerate Specials to full quickly. Assuming Laegjarn/Fjorm meets the Atk check (and they usually will with the amount of Atk manipulation in the build), Bonfire can potentially trigger in one initiation and counterattack, though Ignis is the preferred special if you're dedicating Laegjarn/Fjorm to enemy phase purely. 
  • If you're exceptionally worried about bow matchups, Iote's Shield takes special mention for nullifying flying weaknesses. It should be accompanied by more stat support to make up for what's essentially an empty slot otherwise.