Inferno – “A fitting image for you” [Campaign 5, Mission 2]

Hello everyone. Can anyone tell me if I'm the only one experiencing this issue? I can't seem to get past the first Necropolis town. After capturing Castle-Rock, while my army is weakened, a Necropolis player with a "high" threat level attacks. I replenish my forces and defeat him. Then, I proceed through the newly opened gates and see the Necropolis with a "deadly" threat level. I've tried many things: I waited for a month, building up my army until the threat level was "high," but they still defeat me, both in auto-battle and manual combat. Can anyone suggest how to overcome this? Or am I doing something wrong from the very beginning?