Please offer some advice.

Hello, I'm playing HoMM 4: The Gathering Storm, and I'm stuck on the campaign for Dogwol the Barbarian. The first mission, "Unexpected Loot," is surprisingly difficult – I can't even complete it on the Novice difficulty! Even if I manage to capture one of the fortresses, before I can develop, a red enemy appears from above and immediately defeats me. Are there any veterans who have played this map and can offer advice on how to complete it without using cheats? Thank you.

Added after 10 hours and 16 minutes
Here's a link: http://fcenter.ru/online/softarticles/games/5658... in case anyone else is interested.