With the ability to produce 3 or more Mutation Stacks per second, you become invulnerable thanks to your passive, which gives you 5 seconds of invulnerability every time you 'die', and does not prevent you from gaining more stacks during that time.
This is a lot better than trying to build enough damage resistance with [Adaptation] to survive enemy damage, which is just too unreliable.

Paired Companion build: Vizer Predasite - Tenno's Mecha Hound

How-to/general gameplay loop:

• Cast Virulence(your 1) on groups of enemies to generate Mutation Stacks and generate energy for each enemy hit.

• Cast Larva(your 2) to group enemies for Virulence.

• After building Mutation Stacks, cast Parasitic Link(your 3) on a nearby enemy or ally to double the damage of Virulence and the number of Mutation Stacks produced. Where possible, keep Parasitic Link active on an enemy or ally.

• After building Mutation Stacks, cast Ravenous(your 4), preferably in a good chokepoint. This will heal you and allies, produce maggots(yay!), and cause you to gain approximately 60% more Mutation Stacks.
  • Hold-to-cast Virulence to target maggots on enemies or in groups of enemies for additional Mutation Stacks.

• Whenever an enemy is Mecha-marked, kill it as quickly as possible, applying at least one damaging status effect to it. Doing so will affect all enemies within 30 meters of it with its status effects - of which the damaging ones were already increased twice by Mecha Empowered - which will, without exception, kill anything weak to status effects.
  • Mecha Pulse's effect is granted to whoever strikes the killing blow, regardless of if that is you, your operator, an ally, an ally's operator, or a specter, with companion kills passing the effect on to their owner.
  • The Mecha set bonus will trigger regardless of if you personally kill the Mecha-marked enemy, and damage done by its status effects counts as your companion's damage, allowing it to trigger Mecha Pulse for your benefit.

• Use R⁠olling Guard(tap shift) when in danger(Radiation status, about to cast Ravenous, below 15 Mutation Stacks, after Undying's 5 seconds of invulnerability wear off) to become invulnerable, then use that invulnerability to gain additional Mutation Stacks.

Why this choice/other options:

• No Helminth Replacements - Except for maybe Larva, Nidus wants all of his abilities for consistent mutation stack generation and invulnerability.

• R⁠olling Guard - A core element of the build. Allows you to avoid damage completely, if well-timed, and should scale better than A⁠daptation in extremely high-level content.

• I⁠nsatiable - The other core element of the build. Makes Mutation Stack generation much, much faster, allowing you to easily chain Undying into Undying for survivability.

• Primed F⁠low - Virulence can generate more energy than it cost to cast, which Primed F⁠low allows you to take full advantage of. Having a large energy pool also makes missed Virulence and Larva casts less punishing - with Primed F⁠low(45% Efficiency, 531 Maximum Energy), Virulence and Larva's costs are 11.7% and 7.3% of your energy pool, while with S⁠treamline instead(75% Efficiency, 188 Maximum Energy) they're 26.6% and 16.6% of your energy pool, which is clearly worse - you have to r⁠each at least 150% Efficiency for S⁠treamline to outperform Primed F⁠low for N⁠idus Prime.

• Arcanes - Arcane Energize makes it easier to keep up your energy with 45% Efficiency, while Molt Augmented is great for endurance runs, which this build shines on. Swap to taste.

Tips and Tricks:

• The Mecha set bonus works in several unusual ways:
  • It does not apply all of the status effects at once - if there are more than two stacks of a status, it will apply that status at a rate of two stacks per second, which noticeably extends the duration of the effect in general, especially for Heat status effects.
  • Note that status effects double dip on damage with Mecha Empowered, in the same way as Faction damage mods - which Mecha Empowered stacks multiplicatively with. If you can spare a slot for a Bane, Cleanse, Expel, or Smite mod, or have access to an allied Rhino(Roar is a Faction damage effect!), this can massively increase your damage, and the damage of allies, against Mecha-marked enemies.
  • There is either a bug or something very unusual going on with the damage calculation of the applied status effects - it seems, through some testing, that the damage of each individual damaging status is equal to the total damage of all of the original status effects.
    • This means that, with ten 560 damage Slash statuses, the Mecha set bonus would be applying two stacks of a 5600 damage Slash status every second for five seconds, which lines up with the observed effect of activating it - that everything in range dies extremely quickly.

• When killing a Mecha-marked enemy, try to make sure to be in a group of enemies; Mecha Pulse originates from whoever killed it, and not the Mecha-marked enemy.

  • This applies even if you personally aren't running this build - if you see a glowing Kubrow face above an enemy, being in a group of enemies before you kill it will grant significantly more armor.
  • Killing a Mecha-marked enemy while Mecha Pulse's bonus is still active will override the current bonus with the new bonus, even if that is less armor. This will usually only matter if there are multiple Mecha mod users in a single squad.

• Equip [Amalgam Barrel Diffusion] on your Secondary to slightly increase the usable invulnerability time from R⁠olling Guard and slightly decrease the range your rolls send you.