In the end, I beat it... First — at the bottom of the map, I captured the elves' town,
then immediately — went into the underground, captured a town there — i.e., I eliminated the orange player. I also took another hero, who scurried around at the bottom,
while the main one — (stronger in experience and magic) — was at the top. I built up
both castles, then another hero acted as a courier — bringing troops
from the underground to the main hero. Archers and druids are garbage,
they often refuse to fight..., there's little hope for them.
After that — it was a matter of time — I gathered strength, captured another town —
in the top left corner, then in the right one, assembled troops,
and when I had two armies, I sent the weaker hero with "elven"
troops to the final target, followed, half a move later, by the main hero with
a "demonic" army. The weaker hero was wiped out attacking the final
town, but he heavily mauled the enemy, and then immediately
the main hero attacked and easily won :D