Warhammer 40,000: Dawn of War/Infiltration

Infiltration
This mission begins with an infiltration part where you control just one squad of Scout Marines. Sneak past the first couple of Ork groups until you gain another squad equipped with sniper rifles. You can now move on or backtrack to kill the enemies you saw (keep the other team infiltrated and snipe them from afar with the other; they won't even see you).

Either way, as you move on you will be directed to kill a small group of Orks guarding a Strategic Point. They go down easily as long as you stay far away. Moving on, there's a minor Ork camp that is actually possible to take down with your meager scout "army" but it's probably not worth your time. Sneak past the camp and kill the Ork leader hanging at the ramp as directed.

When the Ork leader goes down, some Space Marines are dropped in once again so retreat your scouts and do the dirty work with the heavy infantry. Move up the ramp and destroy the Ork base. Use your heavy bolters to take out structures from maximum distance like before. Build a base and as many Space Marine Squads as you're allowed.

Now the tougher part; it's quite simple but there will be a whole lot of Orks. You must follow the winding, branching pathways and clear all Ork structures (there are two major bases). Basically you should advance methodically with a flamer squad (I recommend upgrading each squad with one kind of heavy weapon) in front, followed by plasma guns followed by heavy bolters followed by missile launchers. The leading flamers will break morale, making the charging Ork hordes quite inefficient. Plasma/bolter fire will do the actual infantry killing, while the missile launchers hang back to take down any buildings and Killa Kans. If you get 2 flamer squads, 2 missile squads and anti-infantry weapons for the rest, you can take two branches at the same time.

It can be a bit of a grind but just keep pushing on and you should win easily. The Orks will come in hordes but your heavy weapons will mow them down quickly; it's the Killa Kans that can be annoying, needing repeated missile hits and some concentrated fire from your other squads to kill.