IMPORTANT NOTE: As of U27.2, enemies have far, far slower health and armour scaling at high levels. I have read other players calculate that a current level 9999 Bombard is roughly equivalent to an old level 280 Bombard for EHP. As such, the formulae in this post are no longer accurate, though armoured enemies still easily eclipse unarmoured enemies for EHP. Additionally, Viral and Corrosive procs have been altered, Viral being made more powerful and Corrosive being weakened.
REST OF ARTICLE
In my posts and indeed in much of Warframe's content, a lot of fuss is made about armour, specifically enemy armour. Things like veterans always recommending Corrosive Projection as an aura, and defaulting to Corrosive or Viral + Slash for elemental setups. It is not immediately obvious to newer players why armour is so important. With the help of Warframe's always useful Wiki and some maths, I hope to give a better idea of why armour is the one of the biggest problems at high levels.
TL;DR: The effective health of unarmoured units grows at a quadratic rate with level, while the effective health of armoured units grows at nearly a quartic rate. Armoured unit effective health grows far faster than unarmoured unit effective health, thus at high levels, armoured enemies are far, far tougher.
Corrosive reduces both types of armour with its status proc, and is super effective against one type of armour. Radiation is super effective against the other but does not strip armour. Viral + Slash/HM uses Slash procs to bypass armour entirely, dealing damage directly to health.
