Complete the Forest mission (your first encounter is at the beginning of the mission), then return to Pascal. After, you should see him in the City Ruins near the desert (his icon is a smiley face on the map).
****SAVE FIRST**** I had the game glitch on me where I exited his shop but didn't get the achievement (took four attempts). I exited the game completely, loaded it, and tried again, unlocking it.
Next, go over to him, and you'll likely get attacked, smacking him during the fight. If not, have your pod shoot him to stop him. Finally, speak to him to enter his shop (no need to purchase anything), and the achievement should unlock once you exit the shop. If not, reload your save and yry again, though you can also probably just purchase it after your third playthrough, sooner if you use the cheat that takes you passed the third playthrough.