Will probably happen anyway from playing however,
Using the light class will make it easier as you have hardly any health in comparison to other classes
Make sure you have frag grenades equipped in one of the equipment slots( other explosives might work like the pyro grenade )
Going into the shooting range works for this as mentioned in the comments and is a better way than slightly throwing a game , just throw the grenade near a wall at your feet until your die for the achievement ( to select the Grenade is
)