Arcanes:

Molt Augmented: consider it a must, as it get her Strength to 204%.
Arcane Precision: more damage on Secondary weapon is amazing, considering her passive gives them +200% Crit Chance, and a Companion can give +1.2x Final Crit Damage with [Tenacious Bond].
Arcane Velocity: more Fire Rate for Secondary weapons, because why not!
Arcane Pistoleer: infinite ammo for Secondary weapons, assuming you can get a headshot kill (does NOT work on Incarnon forms).

Shards: 2x Duration, 2x Parkour Velocity, 1x Casting Speed.
Tauforged Casting Speed is borderline mandatory; the rest are up to preference.

Nourish build (requires a Forma on [Adaptation] D Polarity, or Omni Forma):
[Brief Respite]; [Primed Sure Footed].
[Transient Fortitude], [Primed Redirection], [Adaptation], [Umbral Intensify];
[Primed Flow], [Stretch], [Augur Reach], [Loyal Merulina].
Arcane Velocity; Molt Augmented.
Note: This build is using Arcane Velocity instead, because Nourish is adding damage already, and Fire Rate is nice to have. Arcane Pistoleer also works, if you can get a headshot kill.

Meruline [Guardian] build (requires a Forma on [Blind Rage] V Polarity, or Omni Forma):
[Brief Respite]; R2 [Power Drift].
[Primed Continuity], [Primed Redirection], [Blind Rage], [Adaptation];
[Primed Flow], [Stretch], [Augur Reach], [Merulina Guardian].
Arcane Aegis; Molt Augmented.
Note: [Power Drift] has to be R2, because of polarities. If you want to forma other slots to max it (or Omni Forma on that slot), then go for it. This build uses Arcane Aegis, since you no longer need Arcane Velocity. If you're using Eclipse, you can go with Arcane Precision or Arcane Pistoleer; if you're using Nourish, you can go with Arcane Pistoleer or another one you prefer.

[Surging Blades] build. I recommend Roar on her 4, to increase the damage and Slash damage her 3 does:
[Brief Respite]; [Power Drift].
[Primed Continuity], [Narrow Minded], [Blind Rage], [Adaptation];
[Equilibrium], [Augur Secrets], [Intensify], [Surging Blades].
Arcane Aegis; Molt Augmented.
Note: You can improve this build with more Forma to slot in [Umbral Intensify] or [Transient Fortitude], but I don't like it enough to do that.