An enemy takes the damage times (100% minus its protection), rounded down. Its protection is its armor against physical damage, its magic resistance against magic damage, half its armor against explosive and electric damage, and 0% against true and poison damage.
Damage types
Reduced by the target's armor.
Reduced by the target's magic resistance.
Reduced by half the target's armor (75% of it for the Royal Catapult, Dwarven Culverin and Ironbark Treant basic attacks and the Perfected Formula blast with the Shrapnel upgrade); magic resistance does not count. The info bar shows it with the physical icon.
Never reduced: armor and magic resistance do not count.
Reduced by half the target's armor, like explosive damage. Dealt by the Tesla X104 of Classic Rush; the info bar shows it with the physical icon.
Not reduced by armor or magic resistance. Dealt by poison effects such as the Rangers Hideout's poison.
How armor and magic resistance work
Armor and magic resistance are percentages. Protection always counts between 0% and 100%: a target with 100% armor takes no physical damage. Armor does nothing against magic damage and magic resistance does nothing against physical damage.
Units (soldiers and heroes) use the same protection, but the blocked part is rounded down instead of the damage taken, so a unit can lose 1 point more than an enemy would: 21 physical damage on 30% armor takes 14 health from an enemy (21 x 70% = 14.7, rounded down) and 15 from a unit (6 of 21 blocked).
Worked examples
An arrow deals 16-26 physical damage; the Troll Champion has 30% armor. A 21 roll: 21 x 70% = 14.7, so it loses 14 health.
A bolt deals 36-66 magic damage; the Necromancer has 60% magic resistance. A 51 roll: 51 x 40% = 20.4, so it loses 20 health.
A shell deals 80-120 explosive damage; the Dark Knight has 80% armor, half of which counts: 40%. An 88 roll: 88 x 60% = 52.8, so it loses 52 health. With Shrapnel, 60% counts: 88 x 40% = 35.2, 35 health.
The Dark Slayer has 100% armor. Every physical hit deals 0 damage; explosive damage meets half of that armor, so 100 explosive damage takes 50 health.
Moglok-Hai carrying his shield has 98% armor and 98% magic resistance. The Fat Boy deals 2,000 true damage, and he loses all 2,000.
Explosive damage and Shrapnel
In battle, Shrapnel makes the Royal Catapult, Dwarven Culverin and Ironbark Treant basic attacks and the Perfected Formula blast meet 75% of the target's armor instead of half of it; the Powerball blast, the other artillery skills and the Ultimates still meet half. The Alchemist Shack's basic attack deals true damage, which armor never reduces, so Shrapnel does not change it. Against an enemy with armor it deals less damage than without the upgrade; against an enemy without armor it changes nothing. The upgrade text reads "Artillery attacks now ignore 75% of enemies' armor."
Ignoring armor and reducing armor
An attack that ignores armor counts the target's armor lower by that many percentage points for that hit only; the target keeps its armor. With his talent Sharpened Rounds, Bolin's basic attacks ignore 25% of the enemies' armor: against a Dark Knight (80%) a hit counts 55%.
An armor reduction takes percentage points off the target's armor: a Dark Knight's 80% armor reduced by 5% becomes 75%, not 76%. Reductions add up, and armor never goes below 0%. Magic resistance reductions work the same way.
Outright kills, eating and dodging
Kills outright
An effect that kills outright takes all of the target's remaining health, whatever its armor and magic resistance. The Fat Boy kills outright every enemy with less than 2,000 health.
Eaten units
J.T. and Sarelgaz eat units. An eaten unit dies whatever its health and armor.
Dodge
Wulf, Worg, Tainted Wolf and Demon Hound have a chance to dodge each melee attack; a dodged attack deals no damage. A stunned enemy does not dodge. The chance is on each enemy's page.