An easy place to get 5 enemies to kill each other is in the "Thornfelt Swamp". In this section you will find a couple of plants that are covered up. Make the first plant shoot down to free the second one. These are the two that you need for this achievement.
Jump back up and go right so they disappear from the screen. Now go back and quickly jump on the lower plant hole and jump back off. The plant on top will shoot at your last position and destroy the plant that is coming out of the ground.
Repeat this until you have 5 kills. The plant will respawn when you jump back up on the right platform.