Kill any boss, that is an enemy with a big red health meter in less then 1 minute. The boss during the mission "Warp Spider Exarch" (mission 6 or 7) is a good candidate for this achievement. When a reinforcement group appear grenade them, so all your squad can focus their fire on the boss rather then regular enemy troops.
You only need to be fast beating the boss the mission itself can be done as fast or slow as you want.