Proto (you'l find him in a garage near the drive in movie theater) will ask you to charge his core. You will find the charge station under the street in front of the drive-in (ride down one of the two exo lifts, in the connecting walkway open the door* and charge the core).
Give him the charged core and the achievement will unlock.
* opening the door might not be available until later. Myself, I found this door only after beating the final boss of the DLC and bactracking. It might have been locked the first time I was there.