In Their Sleep - Call of Duty: Black Ops III

In Campaign, Kill 5 Stunned Enemies in under 3 Seconds - 10

This can be done in any mission, but it is easiest in Mission 6 - Vengeance.

You need continue through the mission, going through the building where the 3 enemies are knocking down the door, getting back onto the streets. There will be a small group of enemies which will huddle around some cover. Once there is at least 5 enemies, throw a Flashbang Grenade and kill at least 5 Enemies in under 3 seconds.

Flashbang Grenade