Heh, I didn't read that both heroes needed to be brought to the outpost; when I went underground with Raiglag and jumped over the outpost via portal, naturally the script didn't trigger. I kept going and started cutting through castles, thinking that was necessary. After the second castle, I had 60 assassins left. Then I decided to restart the map, but scout it a bit more for my future walkthrough. In the end, I killed two more castles and a couple of hero garrisons, and finally read that both heroes must be driven to the pass. Once I brought them, I had to fight through the outpost since the second hero doesn't have a portal. Then I killed the main infernal and completed the mission. My army before the outpost consisted of 94 assassins, 5 non-upgraded furies, 14 minotaurs, 3 hydras, 2 mistresses, and 3 twilight dragons. I killed everyone without any problems. So the mission is elementary; for the Dungeon, troops aren't the main thing.
P.S. The most annoying part is that they never gave the hero luck throughout the entire campaign.