Whilst the only ability you need to unlock in order to get this trophy is Shield Rush, I would recommend getting Shield Rush I if you try to farm the trophy. This way, more enemies can be beaten with less rushes.
The easiest place to do this is in Central Research (Research Sector). Starting from the Control Point, turn left and drop to ground floor, here a bunch of enemies will most likely spawn, here is when you the ability comes in: hold L1 and then press circle while aiming at an enemy. Once you've defeated all enemies in this area, go up the stairs and repeat the process. When there are no more enemies left, quit to main menu and repeat the process until you reach 100.
Here's a video demonstrating the ability: