The most challenging campaign.

Lichs, I completed it. I played like this: with Agrael, I captured the first castle, hired Wingael and Anwen. Agrael gathered resources and then stayed in the Inferno castle until I built it up; with the Elves, I traded resources and raided enemy caravans. The week of Elves helped (Wing is strong in battle, and Anwen is good at defense). Then, when it became clear that things were getting serious, I gave all the troops to Agrael for experience, and with the Inferno army, I cleared out the castles (logistics + luck and something else... but I definitely focused on the Gates of Hell).