Once you acquire the Head of Helios, you can immediately begin working on this trophy. You can work on it as you play through the game, or you can even complete it with the enemies that appear in the room where you acquire the head.
You can blind enemies in two ways. You can hold L2 + tap "triangle" to flash blind a single enemy for a short time, or you can hold L2 + hold "triangle" to charge the head. When you release "triangle" it will cause a large blinding flash that will blind all enemies in the area for a longer duration, as well as light up the area for a while. Either way works.
Just spam either of these methods on enemies until you reach 100, and unlock your trophy.