Limstella

Weapon: Morph Fimbulvetr

Assist: Positional Assist

Special: Glimmer 

Passive A: Atk/Res Unity / Death Blow / Atk/Res Ideal / Atk/Res Push 4 / Atk/Def or Atk/Spd variations of aforementioned skills

Passive B: Lull Atk/Res / Special Spiral / Lull Spd/Res / Atk/Res Link

Passive C: Joint Drive Atk /  Atk/Res Menace / Time's Pulse / Res Smoke / Flexible

Sacred Seal: Death Blow / Atk/Res Form / Atk/Res Solo / Atk/Res Bond / Blade Session

• Morph Fimbulvetr is a little neat package of great damage-increasing effects, including a nice basic Atk +3, and whenever Limstella engages combat three spaces near an ally:
  • Cuts down the enemy's Atk and Res by a whopping -8 in combat
  • Stacks even more damage based on their allies' total Atk/Res bonuses — under reasonable and optimal circumstances like regular Tactics boosts, this should come out to an additional +12 damage. 
• Basically, as long as Limstella cozies up near some teammate unit — and said teammates wouldn't mind taking the effort to buff their Atk and Res and getting their HP shaved by 15 whenever Limstella completes combat — they can threaten enemies with high-damage hits while doing a reasonably good job at tanking incoming hits, also thanks to Morph Fimbulvetr's follow-up blockage. 
• On a build focused on building maximum damage, high Atk supplements are pretty obvious. While simply running Death Blow in both slots will mostly do given how accessible and easy-to-use the skill is, it may be better to double-dip in Res or Def with an appropriate Ideal, Unity, Form, and / or Push skill to enhance Limstella's tankiness while being more functional in both Player and Enemy phase. 
  • For the ballsier max-investment builders out there, running Atk/Spd variations of these skills also isn't an absolutely terrible idea (lookin' at you, Speed meme Sonia), especially given the high-quality Atk/Spd Sacred Seals, like Atk/Spd Solo and Blade Session, easily available to anyone. This takes into account how many new threats dump their Speed in favor of other stats, how many more kills Limstella can secure with a follow-up hit, and how Limstella starts out at a middling but easily stackable 23 Spd. As an obvious disclaimer, don't expect anything magical versus the game's various real speedsters. 
  • On a budget, Atk/Res Link serves to strengthen both Limstella and their allies when a movement assist is used. The visible bonuses earned from this also conveniently feeds into the damage-building component of Morph Fimbulvetr. 
• To further strengthen Limstella's damage, a Res-based Lull or Special Spiral will be ideal. The former always keeps Limstella's natural damage in top shape by denying enemy buffs and debilitating further by -3 in Atk and Res. On the other hand, Special Spiral allows them to cyclically fully charge Glimmer for continued burst damage every match, but requires a bit of setup for its first activation (preferably though precharged means like Time's Pulse and Infantry Pulse support).
• Given how much Morph Fimbulvetr revolves around being near allies, Joint Drive Atk is a natural complement to it, boosting the offense of Limstella as well as nearby teammates. Atk/Res Menace is a more selfish option that maximizes hurt through Limstella gaining self-sufficient buffs while cutting a nearby enemy's Atk and Res. Time's Pulse was mentioned shortly before for making Special charging more convenient and making Limstella less reliant on other allies providing Pulse support. 

Weapon: Morph Fimbulvetr

Assist: Rally Atk/Res+ / Rally Up Atk+ / Other High-Rarity Rally skills

Special: Glimmer

Passive A: Atk/Res Unity / Death Blow / Atk/Res Ideal / Atk/Res Push 4 / Atk/Def or Atk/Spd variations of aforementioned skills

Passive B: Atk/Res Ruse / Def/Res Ruse / Other Ruse skills

Passive C: Joint Drive Atk / Flexible

Sacred Seal: Death Blow / Atk/Res Form / Atk/Res Solo / Atk/Res Bond / Blade Session / Atk/Def or Atk/Spd variations of aforementioned skills

• This is largely similar to the previous Offensive build, but Limstella has the option of running the Rally + Ruse skill combination to: 

1. Buff the stats of a recipient ally — preferably from Rally Atk/Res+ for the next reason.
2. Subsequently juice up Morph Fimbulvetr's additional damage effect — which happens to be based on an existing ally within 3 spaces with the highest Atk/Res bonuses.
3. Inflict stat and【Guard】penalties on enemies within the same columns or rows as Limstella and their chosen ally, making them much easier to kill or approach in imminent combat. The【Guard】penalty also largely makes up for the lack of special denial effects on the build (otherwise present in a high-tier Stance or the underwhelming Guard B slot).
4. Play a more remote, proactive role in Aether Raids and Summoner Duels. This applies to especially in situations where Limstella's contributions may be otherwise limited, such as not being in range for combat or currently having no relevant targets. 

• Limstella is less particular about the variation of Ruse used than Rally, but when it comes to improving their own combat, Res-based skills such as Atk/Res Ruse or Def/Res Ruse would be preferred for increased damage output.
• The previous build's offensive options remain largely consistent here. Again, simply stack up Limstella's Atk with as many high-quality passives you can offer whenever they get into the fray. Choose whichever A and Sacred Seal slots suit your initiation and positioning preferences.

Weapon: Morph Fimbulvetr

Assist: Positional Assist

Special: Iceberg / Glacies / Noontime / Sol

Passive A: Close Reversal / Mirror Stance 3 / Atk/Res Unity / Atk/Res Ideal / Close Counter / Close Foil / Atk/Def or Def/Res variants of aforementioned skills

Passive B: Lull Atk/Res / Quick Riposte

Passive C: Canto Control / Joint Drive Def / Joint Dist. Guard / Joint Drive Res / Joint Drive Atk / Atk Smoke 

Sacred Seal: Def/Res Form / Atk/Res Form / Mystic Boost / Quick Riposte / Bond or Stance variants of aforementioned skills

• A more enemy phase-oriented build than the others, this focuses more intently on Limstella's defensive strengths. As such, we prioritize skills that promote bulk and survivability in two main flavors:
  • If you plan on making a Limstella that counterattacks universally, Close Reversal (or its Dragon-vulnerable counterpart Close Foil or more accessible predecessor, Close Counter) will be their go-to A-slot. Limstella should leverage the newfound ability to combat melee enemies by using Def-fortifying skills such as Joint Drive Def, Def/Res Form, and such. 
  • Limstella can specialize in ranged combat instead, which means skills that enhance some combination of Atk, Def, or Res are best used here. In particular, Atk/Res is preferred across all Stance, Unity, Forms, or Ideals you use to specialize further against magic enemies. 
• Across all builds, you'll be pondering the choice between Lull Atk/Res or Quick Riposte in the B slot. From a stats standpoint, Lull Atk/Res always primes Limstella's bulk and damage by neutralizing then weakening any Atk/Res buffed enemies they come across, while the added follow-up counterattack from Quick Riposte should aid in dealing better damage and securing kills. 
  • Note that Quick Riposte can be included as the Sacred Seal instead to make room for Lull Atk/Res, and that having it anywhere on the build is almost non-negotiable due to the occasional dubiousness of Limstella's low combat Speed. 
  • Running Quick Riposte in the B slot is not only less resource-heavy (as it's more reasonably accessible than Lull), but also gives Limstella room to run an even more potent Sacred Seal such as Def/Res Form for stronger stats or Mystic Boost for added healing.
• The C-slot is largely flexible.
  • For the sake of optimization, Joint Drive Atk, Res, or Def will bolster Limstella's stats in your preferred way while also encouraging you to keep up Morph Fimbulvetr's ally nearness conditions. Joint Dist. Guard, in particular, is valued in general gameplay and Aether Raids Defense to boost the ranged tanking power of Limstella and benefitting allies.
  • Cheap and good old Atk Smoke 3 will also do as a low-investment option that primes Limstella better for multiple fights in one turn.  
  • Finally, Canto Control is a pretty awesome — if not a bit rare at the moment — skill that puts severe movement limitations on enemies with Canto effects. With Limstella's increased defensive prowess, they can afford to run it safely as a checking method against most hit-and-run strategies. 

Weapon: Morph Fimbulvetr

Assist: Positional Assist

Special: Glimmer / Iceberg

Passive A: Def/Res Form / Atk/Def Form / Atk/Res Form / Bond or Ideal variations of aforementioned skills

Passive B: Atk/Res Link / Quick Riposte

Passive C: Gap or Tactic skills

Sacred Seal: Atk/Res Form / Atk/Def Form / Def/Res Form / Stance or Bond variations / Tactic, Opening, or Gaps

• Morph Fimbulvetr is still a potent weapon even with lower investment skills, so much of the build shifts focus on buffing skills to cyclically empower both teammates and the power of the tome. To achieve this, simply pick out the various low-rarity Gaps, Tactics, Openings, or other such skills to be used mainly in the C skill and possibly the Sacred Slot. The ones you pick will be up to the rest of your team's stats, preferred buffs, and your preferred distribution.
• Together with some movement assist, Atk/Res Link directly feeds Morph Fimbulvetr's additional damage conditions by granting both Limstella and their chosen teammates Atk and Res buffs. Otherwise, you can run Quick Riposte to improve Limstella's enemy phase performance with an added follow-up hit. 
• Keeping the default Form skill strategy isn't too bad of an idea, considering the high maximum stat yields Limstella gains from them (+7) and how they encourage upkeeping Morph Fimbulvetr's ally nearness conditions. Limstella's base Des/Res Form will do in a jiffy as it improves both defenses, but if you prefer more damage, running an Atk-based Form, Bond, or Ideal is the way to go. This similarly goes for the Sacred Seal to maximize stat gains.