Go to Chapter Select and pick Chapter 4 - The Dig, and load it up on Very Easy difficulty. Progress a very short ways into the level until you get to the shed where you get the grenade tutorial, next to the boxes of grenades. After the cutscene, skip through the tutorial and run out toward the guys coming at you. Equip your AK-47 and gun doen the guys coming toward you WITHOUT using the L1 button to aim. Take your time and don't try to rush. On Very Easy difficulty, it's nearly impossible for the enemies to kill you,and they will go down after only a couple of hits.
About 20-25 enemies will spawn in this area, but don't worry if you don't get it the first time through. Just Reload Checkpoint and go at it again until you've killed 20 enemies in this manner.
Note: The trophy will unlock as soon as you get the 20th kill in this manner.