Armor Mechanics

StarCraft 2 Armor
Armor in Night of the Dead does not function like it does in Vanilla StarCraft. In StarCraft II, 1 point of armor shaves off 1 point of damage from any source, and only 1 point regardless of how much or how little damage the source actually inflicts.

NOTD Armor
Armor in Night of the Dead however works instead on a dynamic basis, which factors in what you are being attacked with. Some attacks are affected heavily by Armor Values, while some are not. As such 10 points of armor doesn't mean you are taking 10 less damage from attacks, but instead can vary greatly by enemy.

Shield Armor
In NOTD Armor and Shield Armor are two different things. Many enemies deal the same armor reduction against shields, but some deal far less. Shield Armor is separate and is in no way affected by C2, C4, or C6. Shield armor can only affected by a few very limited items and skill. Once a shield is down, any attacks taken are under the effect of normal armor. On a final note, shield armor is still effected by the minimum damages associated with Reactive Armor and Kinetic Armor.

Spell Damage
In StarCraft II, any ability that deals damage inflicts "Spell Damage". Spell damage is unaffected by armor. In NOTD this includes every offensive skill from the humble grenades to the tactical nuke.

Examples
Below are some example enemies and how they respond to armor.

Slashers
Slashers have a very high base damage, but they also have an armor reduction of 11. This means they lose 11 damage for each and every point of armor. So a high armor tank will take few low damage from them, while a lightly armored player takes enough damage to die in 1 or 2 hits.

Agrons
Have a much low armor reduction of only 2. Again, this means they lose 2 damage per armor. While they can be tanked successfully by high level tanks with their Tier 3 skills, the high amount of Open Wounds they inflict is reason alone not to.

Infested Marines
With a base damage of 15, and an armor reductions of only 1, Infested Marines are barely affected by armor. Combined with their fast attacks and high chance to inflict Ailments, these enemies can easily wound or kill almost any player caught unprepared. Avoid tanking them until Tier 3 skills are leveled up.

Banelings
Banelings are almost wholly unaffected by armor. And 3-4 of them will bust you into death regardless of high hitpoints and armor. Only with Reactive Armor can they be tanked, although in large numbers they are still painful. Because of the fact they void the Distinguished Service Medal, it is best to always avoid getting hit by Banelings or Beastlings.

Eos
Eos does terrible, terrible damage. 90 per hit, with a double tap on her attacks. However armor sharply decreases her damage, by 5 per armor. Tanks with excess of 18 armor are almost totally immune to her attacks. Eos can also be tanked with Reactive Armor, which will require much less armor overall to absorb her blows.

Erebos
On Normal Difficulty or Higher, Erebos will bypass armor altogether. Only Reactive Armor and Kinetic Armor would help mitigate his damage, but KA is unavailable in Easy Company. Even with RA, it is best to kite and stun the boss, rather than tank him outright.

Athena
Athena's attacks are greatly increased by armor. Each point of armor adds 4 damage to her already strong and fast attacks. Because of this it is impossible for any class to outright tank her. Direct contact with her must be avoided during the battle, forcing players to get creative.