The main thing is to first capture the Dungeon town, taking only Druids and Archers with you, and rush to capture Inferno. Then, you need to clear the area around Inferno. Gathering more Cave Lords and Succubi, head towards the northeastern town (around weeks 2-3). Capturing the northwestern town is also easy. After that, it's all smooth sailing - you explore the map, defeat neutrals, build up your army, and head towards the last castle.