Army of Darkness

The Army of Darkness achievement in Starcraft 2 requires you to kill 15 infested structures during the nighttime. Incidentally, the buildings have to be destroyed during the night but can be heavily damaged beforehand.

If you are having problems with this achievement do the following: 1)During daytime clear out a nest of most building leaving 7-8 infested structures with some space between them. 2)Bring these structures down to 1/4-1/5 health using careful fire. It is highly recommended to have some heavy and support units(marauders and medics) nearby. 3)Put your forces on hold a bit away from the remaining damaged buildings and await nightfall. 4)Take out the structure, kill off the infested defense force, rinse and repeat.

Although it is possible to get this done in 1 night doing so in 2 is much easier. The mission does not have to be completed in order to get the achievement.