This requires you to get 50 kills with grenades.
If you don't get it by the end of the campaign then i would suggest to go to missions, chapter 2 - mission14, and once in game purchase 2-3 crates of grenades.
The reason i use that specific mission is :
1) Its only infantry
2) The enemy infantry are close together and they swarm you all together. So i would suggest to gather as many enemy infantry together and let them chase you, once you see around 20 together throw a grenade and rinse and repeat until all the infantry is gone. Just make sure you throw the grenade in front of them as it takes some time for the grenade to explode and the enemy troops are moving towards you.
If you don't get the achievement right away, repeat the mission and on the second time it will be yours!