Played on Noble and won.
I grouped about two-thirds of my unit into a stack-o-doom strike force, and left the other third around one the mountain passes into Bourne the Gleaming to pick off any units that got through my strike force. The strike force went straight to Bastradam at two moves/turn thanks to the body mage's haste spell. I took the southern route through the desert, though I don't know if that was the best.
Then I sent the two Arcane Barges to Bastradam. Whenever a weak unit like a spectre or hellhound appeared, the Arcane Barges brought it down with fireballs. I also left the Queen of the Line on the tile where the Ira would otherwise spawn. [Homer Simpson voice] I'm smarter than the Devil, I'm smarter than the Devil...
When the strike force started encountering Infernal units, I had the archmage summon Earth Elementals, and the mages Fireballs and Einherjar, to take them out. I didn't attack Infernals with nonsummons units unless there was absolutely no other option (or unless they were so severely weakened that all they'd do was provide experience). If a big stack of Infernals that could've done serious damage ended up right next to me on the end of the turn, I used Blinding Light on them to keep them immobile, waited a turn, and then used more Elementals and Fireballs.
Because I beelined for Bastradam, I made it before they had all that many strong units there and took over the city without a lot of trouble (with help from the Arcane Barges). A few turns after that, I got 100 units and Basium showed up. He turned out to be no help at all, so I took the Stack-O-Doom plus the Bannor reinforcements north, and used Floating Eyes to find the remaining Infernal cities. After that, it's just a matter of Maelstroms and Fireballs to wear them down; none of your units except maybe an Elemental or two should be attacking at less than ~95% odds. I'll admit I saved and reloaded once or twice when a particularly important battle went against me.
Although the scenario text said no Infernal units may get through to Bourne the Gleaming, I let a few through with no ill effect. I think it was because a few of my dead units went on to serve the Mercurians in the afterlife.
EDIT: After reading some other posts in this thread, I get the impression that you now win the scenario after just killing Hyborem himself, not razing all of his cities. In that case, heading straight for Dis (way in the the east of the map) might be a better idea.