The Fall of Voron

What strategy should I use there? I gather all the heroes, but after joining Ossillion through the portal, a horde of barbarians rushes in, and there's no way to hold them back. I even have a 30th-level Raven and a fully equipped dwarven armor, but the army is still three or four times smaller than the first of the rebels. Or should I not explore the dungeon and the lake, but instead immediately crush the barbarians before they gather?