You will need to have the Close Quarters DLC.
This trophy can be unlocked on any map, however, the DLC purchase is still necessary.
First off, you will need to spawn as an Engineer. Customize your class to include a Carbine (shotguns and PP weapons obviously do not count), a pistol of your choice, and a Rocket Launcher (SMAW/RPG V2).
Now just spawn on the map of your choice. I recommend something smaller, so the close quarters maps will do well here. You can also revert back to Operation Metro if you like though. Now proceed and try to kill an enemy with your pistol or rocket launcher, depending on what you are less comfortable with.
Take your time. Place rockets into chokepoints like hallways or staircases, to get easy kills. Sneak up on enemies for pistol kills, and lastly use your main weapon to just be patient and get a kill. If you can manage to get all 3 in one life through a little bit of luck, skill and Trial & Error, you will gain the achievement.
View the video below for some more information and gameplay of me getting the achievement: