This can easily be unlocked while traveling the rooftops of the Shai-Gen territory. Play on 'Tough', equip a rocket launcher and travel from roof to roof. When you find a squad of soldiers of five or more, aim at the middle of them (for maximum splash damage and efficiency) and shoot. Depending on the height you get, you can get off another shot on a second group, which will probably guarantee the achievement. You can also do this during a during the final mob uprising of each gang. There will be plenty of cars and enemies centered around a small area, which will make it very easy.