Warhammer 40,000: Dawn of War/Under Siege

Mission 3: Under Siege
Your objective is, not surprisingly, to destroy the Ork base. The resistance gets a bit heavier, though.

Go check your two defense outposts and capture both SP's your men are standing around. Finish the turrets you got, build 1-3 around the other SP as well. For each outpost, you want a heavy bolter squad, a missile squad, one of your commanders, the Imperial Guard squads you started with, and the turrets (1-2 upgraded with missile launchers). Get an Armoury etc. accordingly and set this up. The Orks will make annoying attacks with Slugga/Shoota Boyz and Wartraks, but said defense will be sufficient. Keep the Guardsmen back, they're not made for absorbing damage.

When your defense is set, you have some options. The Orks have Da Boyz Huts on the left and right, and a Da Mek Shop in the center. If you go right, you can salvage a Machine Cult and get Dreadnoughts, but you don't need it right away. I would go for the Da Mek Shop first to cut down the amount of Wartrak annoyance (some will still come from the main base, though). To achieve this, take 1 missile squad and 1-2 anti-infantry squads. Scout with infiltrated Skull Probes and missile the buildings from afar. You're done.

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