This is in relation to the boss at the end of level 3 -1 , the Fool's Idol.
After you attack them the first time they will disappear and make identical looking clones of themselves appear around the area as well as re-appearing themselves. When you attack the clones no damage is done to the boss themselves and after a few hits the clone disappears.
The trophy is achieved by killing the boss, without hitting any of the clones once. There are three ways to spot which one is not the clone:
1) As the other guide says, the boss fires out the most powerful attack the Soul Ray spell, the other attacks are not as damaging.
2) If you do not kill the smaller enemies in the area, once the clones appear they all turn in the direction of the true boss to worship them so watching their behaviour works.
3) Probably the easiest method, if you approach an enemy and right click to lock on, the clones will show a small red health bar over their heads, the boss will not as they use the main health bar across the bottom of the screen. So lock on, and if no health bar appears you are locked onto the true boss.