This is a
missable trophy that you can get after completing the main storyline.
In the
Vorago Solitude area you will see an exclamation point. Head to it and you will see two guys, one of them being a man named Sterwin.
He will give you the quest
Guardian Hunter, which requires you to go around collecting specific Guardian enemies with a special grenade.
To capture the enemies, destroy their shields and weaken their health, once low on health; toss your grenade at them to capture the Guardian. If you've ever played a Pokemon game before, its exactly like that.
Once you've collected a Guardian of each element type, Sterwin will bring you back to his hut with the Poacher standing in front of it. You will be given the choice to hand the Guardian to the Poacher, or to kill the Poacher.
To earn this trophy:
Kill the Poacher.
A few other Scavs will appear, kill them all and the trophy will pop.