10 0 0Tyrant will first appear *** Spoiler - click to reveal ***after returning to the police station. After dropping down from the roof and putting out the helicopter fire, re-enter the police station hallway with the crashed helicopter. He will move it out the way and stand still for a moment, allowing you to pop him in the head and quickly get the achievement. This is incredibly easy to do as it only requires one headshot and he'll lumber around in a predictable path. This works for all bullet-based weaponry.