A guaranteed way to get this is on Operation Husky. Once you're off the plane, make your way to the green flare to your south-east. Now, follow the streets until you find a slope going up to an Anti-Air emplacement. There will be a large group of enemies past the slope. Simply throw all of the grenades you have at them. As long as you get five kills within a short timespan, you'll unlock the achievement. They don't all have to be from one grenade. Happy hunting!