This is pretty easy. You can get this after you finish Vaporize and Sentinal Perks as per Paul Doyles solution.
This perk is unlocked by getting 50 grenade kills.
When going through Chapter 3 - A New World, make sure you collect 2 grenades. 1 will be under the table with the Watermelons, beside the shed with the Gold Collectable, this is right near the first commander. The next grenade will be upstairs on a crate in the room with the 2nd commander.
Once you clear the area and have 2 grenades, go back to the truck and proceed to the gate. This is where the checkpoint is. Now every time you restart the checkpoint, you will have 2 grenades and be right in front of a room with around 5 enemies. Also keep in mind, if you mess up your grenade throws, thrown back enemy grenades also count, so this is the perfect spot to farm those grenade kills.
Let me know if I missed anything, and I'll fix the solution up!