This can be obtained with relative ease if the team's cooperative enough. Just tell your squad to group up during an ambush and drop the Amp.
The easiest place to achieve this is during the Cargo Elevator mission. After the slow-motion sequence on the elevator, pick a spot, drop the Damage Amp, tell your squad to group up, and start killing the swarm that drop down.
The achievement is awarded to the whole team, not only to the person who dropped the Damage Amplifier.