Tactics for the Swamp (Fortress).

Damage control. Here is the situation: 2 squads of 20*5HP=100HP, damage variance 1.75. I hit, dealing 20*(2..3.5)=40..70 damage, retaliation (6..12)*(2..3.5)=12..30, meaning the variance is already 2.5 times; with armor, retaliation will be 10..26, which to the naked eye is all the same. Therefore, I control losses only by eliminating hits on combat stacks, which is done via attack, offense, archery. What did I miss?

It's one thing when your troops are hit by 20 units with 5 hits each, for example troglodytes; armor savings of 30% are not significant (by the way, the formula there is more complex, but for simplicity)
And it's another thing when you are hit by 5 packs of 30 griffins,
here 30% is already very noticeable. Especially in castles, hives and Dragon Utopia, in objects where avoiding contact is impossible.