Warhammer 40,000: Dawn of War II

Like the description says, kill an enemy while it's fleeing. You will get this while playing the game but if you really want to get as fast as possible. you need to learn how to make a enemy panic. early missions you only have one good method and that's a grenade in the middle of a enemy squad, killing off most of that squad leaving a few stranglers which will panic and flee, simply shoot those before they leave the screen.

later when you get the heavy bolter squad you will frequently cause panic in enemy troops and it will continue to kill those who flee granting you this without effort.