Eliminate the guardians of the capitals.

As already mentioned, it's easy to kill with paralyzers; the main thing is that they can withstand the first attack from a Guardian. Therefore, I won't consider Undead or Legions. I'll just add that the poison of Death significantly speeds up the process compared to Legions.
I haven't taken capitals with Elves or Dwarves; I just haven't had the opportunity.
With Humans, I took the capital thanks to Assassins – poison deals damage regardless, as already mentioned. The main thing was to give everyone +30 armor potions, accuracy potions to the Assassins, and cast a protection spell on them. That's it, the capital falls.
And, in theory, with the help of artifacts that cause petrification and paralysis (the Incubus Horn and the Soul Crystal) or the Witch's Ring, which turns enemies into demons, you can take any army, as long as the hero hits. And the Blade and Skull of Thanatos, again, significantly speed up the process due to poison.