Demon invasion.

it's unlikely you'll succeed by just staying put, and Grobok will arrive soon. I completed it with a rush strategy: first, the standard restarts for the Colossus on the dolmen, then I cleared the forest and mine during the first week and immediately headed to the purple Castle. Its strong hero (usually Krag) would come to my Castle, I would eliminate its weaker units and take both cities. I would kill Krag in the city with my second hero - . I would defeat Sigrif with my hero and then return to the Castle. Around this time, the brown hero (always Haggeesh) would arrive, and I would defeat her in the Castle. Then, I would immediately sail to the brown lands and destroy them. It gets more complicated after that - you need to clear a portal with a hundred level 7 creatures. This requires Darkness, and it's best to build the guild to its maximum in one Castle, sacrificing everything else (anyway, the orange player will destroy it later). Or, you can convert the brown Castle, but that takes longer. The portal leads back to the purple lands; you need to retake its Castles that the orange player has captured and then go underground to storm the garrison. It's easily done with Behemoths and Darkness, but you usually have to kite the last squad with waves. Then, you capture the orange cities - after a week, he loses, and the mission ends. Grobok and other powerful heroes hang out underground during this time because they don't have any travel spells. I finished it on 3/3/4 difficulty.

By the way, the fifth map is quite easy, again, if you don't just stay in the Castle. With a magic build, I finished it a couple of days before the two Necropolises arrived, and just running around and using navigation with a sextant solves a lot of problems.