Necromancer – “Invasion” [Campaign 3, Mission 3]

Not quite. You need to rush past the mages. There are Archmages near the ford by the river, according to my memory. They need to be eliminated. But immediately after their death, one of the orange heroes starts hunting you. Therefore, you need to run into the Dungeon, capture the town there, and stay inside without venturing out. Personally, I had to eliminate two heroes: one orange and one purple. Then, we go up through the Dungeon to the top and eliminate the hero near the Castle. That's it.