In order to unlock this trophy you must first obtain the shield ability, which can be obtained by completing A Good Defense
after picking up the file Home Safe Test
located in the NSC Control Room during the main mission Directorial Override
After gaining the shield, a new abilities tree will be revealed and you'll need to spend six points to unlock Shield Barrage. To trigger the shield burst you must first hold L1 and then release it while aiming at an enemy.
I recommend using Central Research (Research Sector) as the preferred location to unlock the trophy, as the number enemies there is high and they aren't as aggressive as in other sectors.
Here's a video showing you how to use this ability: