This achievement requires that you kill two enemies by firing a single rocket. This is ONLY obtainable in CAMPAIGN. The first location you can earn this in campaign is during Mission 7: Critical Path. The entire mission has you gunning from an airborne drone. The drone's starting gun is a machine gun, but switching weapons will bring out the rocket launcher. Use this and in no time you will surely kill two enemies with one rocket. In my experience, the achievement doesn't always unlock when you kill two enemies. For me it finally popped when I killed at least four with one rocket. Just use the rocket for most of the mission and there is no chance you wont have this by the end.