As an Octavia main, this is my go-to [Octavia Prime], fully maxed and ready to brave the steel path, and destroy everything the game can throw at me. The aura forma is not necessary but purely personal preference, as is the umbral forma. The second arcane slot is flexible and you can use whatever you feel is necessary. If I had to place a preference, I would probably go Arcane Strike, Arcane Nullifier, or Arcane Eruption. Strike for DPS with a melee, nullifier for energy issues, or eruption for the added CC.

As far as Helminth goes, the only ability you could possibly [Sacrifice] is [Octavia]'s 2. What you choose to replace it with is your decision alone.