It turned out there are many mysterious urban legends, in short: Not everyone understands how resists actually work, and which formula should one use for calculations. So let's explain it on some examples.
Example 1., just resists:
20 000 normal damage is applied on death knights
1 death knight has 1350 life, 30 normal resist
Yes, there are 2 ways to look at it, 2 possible formulas how to treat it. (But it is basically the same thing)
1) Resist as a "HP addition towards that particular damage type". So:
1350 x 1,3 = 1755 HP of each death knight, but only towards normal damage
20 000/ 1755 = 11,39
11 death knights are killed by 20 000 normal damage.
2) Resist as a "damage reduction".
20 000/ 1,3 = 15385 of opponent's actual damage in this situation
15385/1350 = 11,39
11 death knights are killed by 20 000 normal damage.
It doesn't matter which way you choose to calculate it.
(20 000 cold or fire damage will kill 14 death knights.)
Example 2., with addition of HP bonus
20 000 normal damage is applied on death knights, with 25% HP bonus (in hero skills, potions and artifacts)
1 death knight has 1350 life, 30 normal resist
1) Resist as a "HP addition towards that particular damage type". So:
(1350 x 1,25) x 1,3 = 2193,75 HP of each death knight, but only towards normal damage
20 000/ 2193,75 = 9,12
9 death knights with 25% HP bonus are killed by 20 000 normal damage.
2) Resist as a "damage reduction".
20 000/ 1,3 = 15385 of opponent's actual damage in this situation
15385/(1350 x 1,25) = ...
15385/1687,5 = 9,12
9 death knights are killed by 20 000 normal damage.
Example 3., "United theory of everything"
If happens everything that can possibly happen, final HP against a specific damage type (X) is:
X = ((basic HP + upgrades) x bonuses sum x S1) x (basic resist + upgrades + bonuses sum + S2)
bonuses sum = potions +hero skills + artifacts
S1 - special abilities with effect on HP (nightmare, almus)
S2 - special abilities with effect on resist (ice fiend, rune smith)
20 000 cold damage (from nightmares) is applied on ice fiends
ice fiend basic HP = 650
ice fiend HP upgrade (3rd level) = 30 x 3 = 90
HP potion (2nd level) = 15 %
"Health" hero skill (7th level) = 14 %
artifacts together = 27 %
S1 ("Aura of despair" of nightmare, -15 % HP) = 85 % of HP
unit basic resist = 0 (ice fiends have normal resist)
unit resist upgrades = 0
hero skill = 0 (cold resist skill available only for dwarves)
artifacts together = 16
S2 ("Frost shield" of ice fiends) = 20
X = ((650 + 90) x 1,(0, 15 + 0,14 + 0,27) x 0,85) x 1,( 0 + 0 + 16 + 20)
X = (740 x 1,56 x 0,85) x 1,36
X = 981 x 1,36
X = 1334 HP towards cold damage
20 000/ 1334 = 14,99
14 ice fiends are killed.
😉 Just ask if you have more questions.