In the seventh level, "The Salvation: the Battle for the Salvation," continue with the level until you reach a long hallway with Scout troopers at the end sniping at you. Don't go to them, turn left into a small room and you'll find small "explosive" spider droids all over the room. Take them out using Force Lightning or Force Repulse. Some of them are well-hidden in that room, look around, behind the broken tanks, everywhere (I found the last one right above the broken fuse outlet). Once you take them out in that room, the trophy will unlock.