This achivement is worth 10 gamerscore in order to get this you must Kill 4 enemies within a single instance of Focus. To do this you must first acquire the Focus skill from the Combat skill tree. Also buy the respective upgrades if you can. They will make this much easier. Cops are easy targets to get this achievement. Shoot around until someone calls them. They usually send 3 cars, with 2 officers in each car. Aim at their windshield as they drive towards you, press the right stick to activate focus, and kill them as quickly as possible.