Tactics for the Swamp (Fortress).

Damage control. Here's the situation: 2 stacks of 20 units each, with 5 HP, damage spread of 1.75. I attack, dealing 20 * (2..3.5) = 40..70 damage, the counterattack is (6..12) * (2..3.5) = 12..30, so the spread is already 2.5 times greater. With armor, the counterattack will be 10..26, which, at first glance, is not very effective. Therefore, I control losses only by avoiding attacks on combat stacks, which is done through attacks, offense, and archery. What did I miss?

The following was overlooked.
When dealing with neutral creatures (I assume that's what we're talking about in this specific case), the situation should be different.
Stacks of 1-1-1-15 (the remaining units have died) against 20. One unit attacks, and it dies. The 20-unit stack moves and attacks the exposed unit. After that, the last unit attacks without retaliation, and the 15-unit stack joins in.
The general tactic for playing against neutral creatures: units to draw part of the stacks to the other flank, units to reduce the counterattack, and units as a target for weakened stacks (which are afraid of being overwhelmed by the main stack). At the same time, it is extremely desirable to take into account the possible morale and mechanically protect your main stack from neutral creatures that may have low morale.