There are a lot of reasons why you want to become super tanky: High level missions, long endless runs or assassination targets can hit quite hard and since you’re not going to have a regenerating shield, you often times need to change your build to survive the incoming damage.

In addition to [Vitality] you’re going to pick [Steel Fiber] and [Armored Agility] to increase your armor from 450 to 915, which will greatly reduce the incoming damage.

Combine that with the great health regeneration from your fourth ability and you’ll be able to facetank even super high level enemies.

In order to still deal a good amount of damage, you do need to get your mutation stacks up to 100 as fast as possible.

Use your third ability ‘Parasitic Link’ to redirect some of the incoming damage to high health enemies and learn to rotate your first two skills to grow the needed mutation stacks. Other than that you simply use all your abilities and progress through the mission.

Polarity Matched / Workable Builds
https://overframe.gg/build/155286/nidus/nidus-the-crowd-control-build/
https://overframe.gg/build/155290/nidus/nidus-the-standard-build/

Source
https://warframe-school.com/nidus-builds-guide/