Uninvited guests.

The demon won the very first siege on the 3rd week. He has upgraded machines that immediately destroyed the gates, and his ballista decimated half of my army. Now that's what I call a special tactic!))))

You need to analyze who is coming "to visit" you. If you see a "Debilka" (likely a nickname for a specific unit or hero), you know she'll bring her plasma cannon. Look for artifacts that provide protection against magic (specifically fire) and don't place your units in a tight group to avoid area-of-effect damage from her shots. If you can't win right away, replay a couple of turns and prepare.

For example, her "special tactic" is completely useless against dwarves because almost all of their higher-tier units are immune to fire. So, to kill one "chicken" (another likely nickname), even a heavily upgraded ballista (three shots per turn) from Debilka will take more than 5 turns, while a "Vitek" (another likely nickname) can take down a couple in one turn.