This achievement can first be done when Tyrant appears.
*** Spoiler - click to reveal ***Upon returning to the police station, you'll drop down from the balcony, and will have to put out the helicopter fire using the valves and pipes. When the fire is put out, walk into the building and he will appear, move the helicopter out of the way, and stand in the hallway. At this point, shoot him in the head to remove his hat.
Keep in mind that Tyrant cannot be killed; only staggered. This is very easy to grab and can be done using an weapon.