I completed the mission using a hero! Here's how: I bought all the defeated enemy heroes. In the end, I had all Warlocks at level 22, and the computer was fighting with Mages and Necromancers. Sometimes, I had to leave some troops in a mine before each Warlock, to reduce losses – it doesn't matter if I deploy 30 or 3 Imps, they all die from the enemy hero's magic. And the enemy army usually doesn't die from my Imps, but from Raileigh's magic. Also, a very important artifact is the Emerald Boots (+50 percent to earth element) – for a devastating Meteor Shower combined with Arcane Wave.