This is one of the more simple trophies in the game. The ignition attack is a super move which takes out a good chunk of the enemies health, even if it's a boss. Filling the red gauge on the left to maximum you will see it say "L2" meaning you can activate your enhanced state. Simply beat up the officer while in this state, and finish them by pressing L2 again while locked on to them or facing their direction in order to execute an ignition attack. If it hits, you should take them out even if they are around 20% health depending on your level.