If you don't get this during the normal game or are playing on an easier difficulty, you can easily get this through the use of Friendly Fire attacks (which will also reward the "Oops" achievement).
An easy way to do this is to go into a town with weak guards like the Rail Nomad Camp and trigger an encounter. Then, have one of your squad members fire a rocket launcher into all but one of your squad, which will likely kill all of them. Then, you can finish off the one guard with your last standing squad member to trigger the achievement.
Keep in mind that on Rookie, Friendly Fire seems to be completely disabled.