[Nidus], or also known as pocket nidus, is a Warframe without shields who stays immortal by stepping on enemies a lot.
His main team-use is, to break the game, by multiplying their modded ability strength with his 3rd ability Link. Which will not just multiply the number you see in your arsenal, but the effective ability strength currently applying to [Nidus] himself and another Warframe.
(Helmith invigoration 200 strength + relic Warframe corruption makes for interesting gameplay, as you multiply your own and partners strength by around 3-5 times their current level)

This build makes sure that you stay alive longer, while still giving a decent boost to health and duration.

Nidus 4th ability can be either swapped out for roar, nourish, xatas whisper or elemental ward fire/ice.

Roar, nourish and xatas gives you so much DPS you could play joke weapons like the [Viper Wraith] if you mod it decent enough.

Elemental ward fire is basically a quick heal for you and allies within your range, also it gives you the option to switch out [Energy Conversion] to [Rage], if you don't own a specter that regenerates you energy, or if you enjoy solo gameplay without consumables.

Elemental ward ice gives you a ton of armour, and is best played with a [Wisp] to gain health over time, to turn off your brain and just shoot dudes with health bar.

Archon Shards are not necessary, but I personally would put 4 red ones and a green one into him.
2 for ability Strength and 2 for ability duration. As well as the green one for more corrosive procs. (Tauforged if just one)
But since his strength is the stat that breaks the game you can do whatever with them. If you want full strip you can instead put 2 normal green shards for more corrosive procs into him instead. But this will do enough for solo gameplay till the mid thousands.

Gameplay:
You find a bunch of enemies, use your 2 to group them into a ball, use your 1 to gain stacks.
Make sure that there are at least 4 enemies in the ball to regain lost energy on cast.

Use your 3 after picking up an orb and shooting an enemy with a status weapon for procing [Growing Power], to amplify another warframes abilities. Or ignore everything and use it on an enemy to gain 90% damage reduction and status immunity.

Use your 4th for either damage support or survivability, depending on what ability you have replaced it with.

Your 1 deals a lot of damage, but your weapons deal most likely more damage if they are modded correctly.

If you have Unairu maxed out, so you can use the operator to strip them of their shields and armour, while they are stuck in your 2nd ability.

If you have the [Laetum] pistol or any other Zariman gun, so you can use your 3rd ability on an enemy and use him to charge the secondary fire ability with that immortal slave.

Also your 1st gives you crit chance on your primary weapon scaling with Ability Strength.