This achievement can be done really simply. All you need is either Xbox Live Gold or a friend IRL. If you don't have Gold, you can have a friend join your game in split-screen, and go on opposite teams. Kill your friend and press either
or
to make a gesture, and the achievement is yours. If you do have Gold, go into a multiplayer match, take someone out, press
or
to gesture, and the achievement will still pop up.