This build is intended to be used with a decaying dragon key equipped

How does this build work?

With a decaying dragon key, you'll have 75 shield. With Brief Respite and 100% efficiency, casting Molt, Toxic Lash or Miasma will fully replenish your shields & re-enable shield gating. Assuming you use a typical primer secondary weapon with [Augur Seeker], the addition of [Augur Reach] + [Seeker] will enable your Spores casts to also get you most of the way back to full.

When you enter a zone you spore, toxic lash, then kill the spore'd target to spread. Try to operate out of the middle of the map so you can manually kill spore'd targets to spread rather than having to miasma. It also helps ensure you don't miss out on affinity from any team mates' kills.

If it's a later zone & you're not the leader, try to not to cast additional abilities early into a new zone until you find out whether there'll be a Host Migration. Most people leave well before zone 8 & when they do your Spores & Toxic Lash will get wiped, and your energy level will stay the same. If you spent extra energy or have been heavily drained you may have to hide for a bit until you can scrape up some energy.

If your shield gets removed while you're out of energy, [Rolling Guard] will give you some temporary immunity. Zenurik will tick back 15 points in its three seconds, which gets you most of the way towards being able to cast Spores & get some shields back. Operator mode is also an escape mechanism to void dash into [Shelter].

[Umbral Vitality] is purely for comfort, but it does give you a bit of survivability leeway which helps when solo. The two umbral mods could be replaced with regular [Intensify] & whatever second mod you prefer. Using [Equilibrium] with a Synthetic Fiber + Synthetic Deconstruct pet would boost your energy supply & potentially let you drop Arcane Energize or Zenurik - you will be more fragile though.

If played properly you should die less than one time in ten, and consistently be able to do a full 8 rounds even if you're solo.
If you have [Primed Sure Footed] you can use it in place of [Cunning Drift] and that'll limit your deaths even further.

Additional Setup I run

Operator: Zenurik, purely for energising dash. The buff will stay on you between rounds so always use it just before going into the next portal.

Operator Arcanes: Magus Elevate (or Magus Repair) for healing, Magus Lockdown is an optional but highly useful emergency CC.

Warframe Arcanes: Arcane Energize is semi-required when solo just due the the sheer quantity of energy drains thrown at you in some maps.
Second arcane is optional, I typically use Arcane Strike for melee attack speed

Weapons: You'll primarily want a good, strong & comfortable-to-use melee. I have a range of [Nikana Prime], [Venka Prime], [Nami Skyla Prime], etc, that I like & have setup with lens' for different operator schools. They're basically all modded base damage (condition overload or primed pressure point), Blood Rush, attack speed (Primed Fury / [Berserker Fury]), [Primed Reach], [Organ Shatter], [Weeping Wounds], [Sacrificial Steel], [Gladiator Might].

Remember how affinity is split. Anything you kill with warframe abilities (e.g. spores) goes 100% to your warframe. Anything killed with your weapon is split 50/50 between that weapon and the warframe. Anything killed by another party member is split evenly between your warframe & all weapons equipped.

Therefore unless you want to put a Lua Lens on everything, you may be better off just using warframe + melee. However that means you can't run [Augur Seeker] via you secondary, so that would be a worthwhile third lens. Equipping a primary weapon just means you'd need to use a fourth lens so it's probably not worth it unless you've got a very good reason (e.g. you're using some sort of [Argonak] based dagger build).

Because this build is intended to consistently & reliably do 8 rounds rather than top the charts on round #1, a less survivable but higher power Saryn can out-damage you in earlier rounds, therefore it's a good idea to hover close to anyone rivaling you on damage to avoid wasting affinity gain.