To unlock this achievement, you need to kill 2 enemies with a single shock dart. To have unlocked the pen gun, first you need to have reached the third mission 'License To Kill', where you will be given the gadget at the beginning. You then need to find a place to use it and a good place to do this achievement, is the same mission where you acquire it. After you have reached Sanchezs' office and you are making your escape, there is a section where you detonate a bomb and all of the water pipes burst. You will first need to kill the 2 enemies with your pistol in the slow-motion scene, but after that 2 more run from a door on your right. If you manage to get them in the water and shoot one of them with the shock dart, it will electrocute both of them and earn you the achievement.
Alternatively, you can run into the room where the 2 enemies come from and there will be a few more enemies and water bursting from pipes. You can try to gather some enemies together in this confined space and kill multiple of them at once, but you have a higher chance of dying because of the amount of enemies present. If you fail to do this, the closest checkpoint is this very area and you can either die or reload the checkpoint form the menu and try again.
Consult the achievement description for 'Very novel Q' for a video walkthrough.