Heroes V: Theory of Power

Friends, can anyone help me with the campaign walkthrough in "The Theory of Power":
New Throne, third mission "Fire Bridges".
It doesn't even last a month from the start of the mission before Inferno arrives with 20 Archdevils and completely destroys everything. I've tried all sorts of tricks, but nothing works. What's the catch?
Hey everyone!
I have the same problem! The advice given is completely unclear...
"and Leva could have just answered... we tried it, and it worked - you need to attack and gather all the Archdevils around you) he was probably afraid to attack... and they immediately took him out"
Please help me:confused: :confused: :confused: