In Act 1 under the Casual difficulty setting, play the very first chapter "14 Days after E-day" until reaching the part where you are about to go outside for the second time--when the Locusts are about to break open the door. Run to the door that is not in front of them or you will die and make sure you have frag grenades equipped. When they break open the door, stick the very first Locust that comes through the door, then stand in front of him so he cannot run away. When the grenade explodes it should kill all three of the Locusts and you might survive. If you survived, make sure that you killed all three of them, then pause game play. Load the last checkpoint and do this ten times. You do not have to survive while trying to get this achievement.
Play the "Evolution" section of Chapter 3. After you rescue the stranded, you will reach a point where you have to cross wooden floor boards that may fall away. Cross all the way to the other side where the respawning grenades are located. Standing just at the edge of the planks, wait until a few wretches gather underneath and toss a grenade in the middle of the group. Refill your grenades as needed. To make it even easier, do this is co-op mode and have your partner stand on the planks while looking down into the pit.