First off, you'll need to build the schematic for the NX-1 Disruptor, which can be found on the bottom floor of the parking garage before reaching the generators inside. Now, craft it by finding 3 red tool boxes, and making it in the skills menu.
The achievement itself is pretty easy, you'll just want to charge the Disruptor using LT, and shoot the large ball which explodes on impact into a group of 3 or more enemies. It will take out most of the small enemy types in one shot.
You can use the Riot Shield or teammates to help you group the enemies together as well, before taking the shot.