You'll get a few of these through normal progress, but if you're looking for an easy way to get them quickly, load up Act 5, Part 2
. From the beginning, kill the 2 enemies and take the cargo elevator down. At the bottom, peak around the corner and shoot the red tank next to the 2 enemies inside. Once it blows up, both enemies will get killed and you'll get credit towards the achievement. Now, pause the game and "Restart checkpoint", to respawn at the bottom of the elevator with both enemies alive. Rinse and repeat until you unlock the achievement.