Warhammer 40,000: Dawn of War II

After winning the mission 8 "The Defense of Argus Gate" you get this achievement, keep in mind that the enemies at the end of the mission will keep coming in waves without end, so focus on your task instead of keep shooting them. You need to fight your way through the hordes, you can't just kill enemies until there is no more and then go forward. because there will always spawn a new group.