This achievement is unlocked after you first view the inventory in Emil's shop. After you look at the inventory, whether or not you buy or sell anything, Emil will say a few things. The achievement will unlock as Emil is talking.
The earliest you can unlock the achievement is during your first playthrough right after you encounter Emil during the Forest Kingdom main story mission. You will not have the opportunity to interact with Emil during your first encounter. Later, after you talk to Pascal to end the Forest Kingdom main story mission and initiate the Report on A2
main story mission, though, Emil's shop will appear as a white smiley face on your game map of the City Ruins. You do not need to unlock the achievement at this early opportunity. Emil will be around for a while and will reappear during the second playthrough. It is possible that he will disappear for good. Nevertheless, his disappearance cannot be triggered without unlocking this achievement so this achievement is unmissable.
Emil is in a roving vehicle near access points. Since he is near access points, make a save before interacting with him. Locate his icon on the map and then go towards it. You might have to adjust your route because he is moving so consult his map if necessary. You are close when you can hear the music he plays. You interact with him by attacking him--a ranged attack is easier but a melee attack works just as well. If you miss, he will come back or you can move to intercept his route. Shooting him causes him to stop. He is not much the worse for wear. He will remember you from the 'previous day' and ask you if you want to buy something. Look at his inventory and optionally buy or sell stuff. When you exit the inventory screen, he will say a few words. The achievement unlocks as he is talking.
If the achievement does not unlock when Emil finishes talking, you most likely have experienced a glitch. In the event of a glitch, clear the cache. To do so, turn off the console by holding the
button on the console for at least ten seconds and then unplug the console for at least 20 seconds. Load the save you made at the one of the City Ruins access points before going to find Emil. Clearing the cache will most likely remedy the problem and the achievement should unlock when Emil starts speaking after you look at his inventory.