A good level to get this achievement is on level 6. To get this achievement you must have a Beserker explode and kill 2 other enemies. These enemies are the bigger/tougher enemies that have lots of life and run and explode near you when they are low on health. When you reach the excavator and power it on, you will have to survive a massive onslaught of enemies attacking you. Two of these enemies are Beserkers which will come running at you and then explode when you hurt them enough. Just focus all of your fire on the Beserkers and don't harm the creepers and you should be able to get this. There are a lot more of these creatures throughout the game, so chances are you will get this without trying for it.