Warhammer 40,000: Dawn of War/Under Siege

You start this mission with a few squads and a basic base. Build an Armoury and some more Plasma Generators. Start making more Space Marine Squads when you can.

In the beginning you are holding two defensive positions some way from your base. Move your Force Commander and Librarian over to help defending them. Start finishing the Heavy Bolter Turrets by the eastern position while you capture both Strategic Points. Build a Listening Post at the eastern position and send the Servitor to build a Listening Post and some Heavy Bolter Turrets at the western position. Use a new squad to capture the Strategic Points between your base and the outer positions and LP them when your Servitor has time.

Your defensive outposts will be attacked by basic infantry and vehicles (Slugga Boy Squads, Shoota Boy Squads and Wartraks). One Marine Squad, a commander unit and the Guardsmen per outpost should be adequate for now; just keep the Guardsmen back and let your Marines soak the damage. The enemy infantry will die easily but Wartraks can be a nuisance if 2-3 come at once, so it's probably easier if you upgrade some of the turrets with missile launchers and try to get a missile-equipped Marine Squad to back up both positions.

When you have a solid defense set, you can start clearing out the city. There is a stray-dropped Machine Cult a bit east of your eastern outpost that you can get right away, but you don't really need vehicles yet. 1-2 anti-infantry squads with 1 missile squad should be quite sufficient to destroy the Ork base in the center and stop that constant Wartrak harrassment. Push north, capture all Strategic Points and post a couple of squads by each of the three ramps leading into the city. There will be some Ork attacks from their main base up north, but they shouldn't be very dangerous. Remember you can build turrets around the top Strategic Points if you need extra firepower.

When the city is empty, build 3 Dreadnoughts (you can upgrade them to ranged weapons, but they'll deal more damage if you leave them in the default melee configuration and back them up with ranged infantry), follow them with 2-4 Marine Squads (preferably heavy bolters and missiles for long range) and trample the main Ork base. Bring a Servitor in case you need repairs, but the Orks will most likely be dead and gone before that's necessary.