Regarding the AI's behavior.

I tried playing this way against neutral creatures. I created one additional unit besides the main one, with only one creature. It didn't help much; in the case of archers, the neutral unit attacks the unit with more creatures.

You can use these single units to block archers so they don't shoot. Approach them closely and don't attack. They will still shoot once, but only once.

Could you elaborate?

For example:
1 peasant attacks 50 hamster berserkers. The hamster berserkers are all alive and kill the peasant with a counterattack.
10 swordsmen attack 50 hamster berserkers. They kill 30, and there is no counterattack (they attacked the peasant).
20 hamster berserkers attack the swordsmen, kill one, and die from the counterattack of the remaining 9 swordsmen.
As you can see, this tactic saved you at least one swordsman who would have died from the hamster berserkers' counterattack.