If your starting unit is strong enough (and it should be by now!), send it at once to (1) to take back Damanhur. You'll have to defeat three Genies for that. A second hero would be useful here, especially the one with Snipe (Carlini, Arri). Don't forget to expand your base meanwhile, starting with getting a second mine.
When you take back Damanhur, you'll probably be very weak in numbers. Head back to your base and gather strength for your next move. No one will attack you now, so there's no need to hurry. Small Dark Alin forces will try to get you from the East, but even ordinary infantry is enough to stop them. Get a second city (2) if you feel like it.
HINT: There's a Vinci mine to the West (3). Miners there will tell you to help them with Rukhs that attack them from the North (4). When you take care of them, they will refuse to help you in return, so kill them all and take over their mine.
Your final attack is a piece of cake. Just accumulate enough forces and destroy everything that stands on your way. The only real threat is the Fire Golem (5). Treat him with your sniper heroes' abilities. Taking over the Citadel (5) will end the mission.