This achievement can be quite annoying. Some Androids are simply invincible. You will need to kill the first or second Android that you come across in the Lower Levels, after you have received the Pistol. An Android will take 5-8 shots before it finally dies. My best strategy for making this happen is to stand by a doorway that you can easily get in and out of, fire off a couple of shots and then disappear back into the door. Come back out immediately and do the same thing. Rinse and repeat until the Android is dead.