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Any advice for Chapter 4 in the Empire Saga? Its the one where you have to control 60% of the map. I can clear the northern area easily but as soon as I kill the demons guarding the pass to the south I get swarmed.
It wasn't easy but I was able to power through. Took a lot of turns just stockpiling resources and building up an army.
What I did was accept the Alliance with the Mountain Clans at the start of the scenario, then let their troops go south through the pass first to serve as cannon fodder by blunting the initial assault. It's important to capture the city near the pass and use it as a bastion/rest stop for your troops. The Legions seem very overconfident and like to attack you even in cities, so take full advantage of that 30% armor boost from a level 5 city. (Pre-cast Holy Armor on your hero as well for a massive 50% armor boost that more often than not, when combined with a Patriarch unit, lets you easily outheal any damage during the fight.)

Once the initial rush of the Legions troops have been thinned out, you can start making forays into the southern area and claiming cities one at a time. If you can afford it, the hireable Dragons from the mercenary vendor make for a very powerful army too.
If I'm correctly remembering which scenario this was, Fire Ward and Mind Ward help when it comes to surviving the spells they start throwing when you start to pass through the bottleneck. Decoy thieves are good too as the AI always seems to set an unreasonably high priority on destroying thieves when it decides who to cast spells on or attack.