Wetwork Achievement in Halo: The Master Chief Collection

On this map, you will come to notice bright red/orange devices that are hanging on the ceilings of each base. When shot, the devices will be destroyed and release a waterfall that blocks the lines of sight through the openings of the base. This effect will only last for about thirty seconds before reverting to its original state.

This can easily be done in a Custom Game with another player. When the match begins, you will need to personally destroy the device inside your opponent's base and grab their flag before the waterfall vanishes. The achievement will unlock as soon as you exit their base with the flag.