After trying different combinations of Range/Strength/Efficiency, this is my absolute favorite all-round build. Lots of people have independently come up with this build, which I think says a lot. I think all the Formas are safe for a variety of different builds if you want to experiment yourself. You need to add one extra Forma to spec it for Steel Path.

Build

You do need to drop one point of mod capacity (mine is in [Umbral Vitality]) to fit it all with four normal Forma. Dealer's choice between [Cunning Drift] and [Streamline].

• Max range is a must for me ([Overextended], [Stretch], [Augur Reach], and Cunning Drift).
• My personal preference is 130% efficiency (Streamline) so I can easily do the combo of: Toxic Lash -> Spores (+hit) -> Miasma (105 energy total).
• [Umbral Vitality] and [Intensify] combo to maximize survivability and mitigate the strength loss from [Overextended].

• Duration ([Primed Continuity]) primarily for Toxic Lash usability, but also helps Miasma's total damage output and slightly slower Spores decay.
  I like [Energy Siphon] for its QOL but it's certainly not mandatory, especially with Zenurik school. Corrosive Projection** is a common alternative.

• Last slot is flex:
  -I like [Transient Fortitude] the most to increase my damage output. I wouldn't do this without Primed Continuity because I like the extra duration.
  -You can mitigate energy issues with Hunter Adrenaline (or Rage)
  -You can try [Energy Conversion] to keep a higher duration while still maintaining some strength with energy pickups. I prefer just [Transient Fortitude] though.

I like Arcane [Guardian] and Arcane Grace for their survivability. You can safely drop Grace for an Arcane Energize, but I'm poor. Healing Arcanes on your Operator help immensely.

To spec for Steel Path you can move [Augur Reach] into [Streamline]'s spot and Forma the vacant slot for [Adaptation]. You need the added survivability. If you use an Umbral Forma then you can leave it as a flex spot. Having default efficiency is fine in Steel Path since the increased spawn rate makes it easier to maintain spores.

TL;DR: I prioritize Range > Efficiency > Duration > Strength

Usage

Pretty straightforward. I try to keep Toxic Lash (3) on at all times. Cast Spores (1) on a target, and then either hit it if Toxic Lash is activated or kill it otherwise to spread. When you've spread a lot of spores, cast Miasma (4) to nuke everything. Efficiency lets you spam the 3-1 combo as needed. At low levels Miasma will just kill on its own. If you start to take damage or want to drop a status then use Molt as your panic button.

I have no issues clearing SO, ESO, and Sorties. With Zenurik school it handles Energy Reduction just fine because of efficiency. This build, as is, can do Steel Path but you can't tank much without [Adaptation] (refer to note on how to spec for SP).

Compatible Builds

Compatible with my meme build: Melee Auradin Saryn (Venom Dose + Eternal War)