Section 8: Prejudice

This trophy is not hard at all, just requires a little bit of patience. You can get this trophy in level 2 of the campaign where you have to clear out sections of an enemy base for your teammate. It requires two shots to kill an enemy, regardless of a headshot, so you can shoot the enemies shield away by headshot or not and then carefully aim for the head and fire. A notification at the bottom will pop up when you get a headshot and if you finish before you get 20 simply restart and you will keep all the headshots you already have.